Military uniform
The French grey was probably decided upon, like the Austrian grey, as being a good "service" colour, which could be cheaply manufactured.

During the 18th century the normal military uniform in Europe comprised a standardised form of civilian dress (tricorn hat, long-skirted coat, waistcoat and breeches). One distinctively military feature were the long canvas gaiters which came up to mid-thigh and had multiple buttons. Dress was surprisingly standardised between European armies in cut and general outline. The distinction normally lay in colours: red coats for the British and Danes, light grey then white for the French, Spanish, and Austrian infantry, dark blue for the Prussians and Portuguese, green for the Russians, etc. Within each army different regiments were usually distinguished by "facings" — linings, turnbacks, and braiding on coats in colours that were distinctive to one or several regiments. The Royal Comtois Infantry Regiment of the French Army, for example, had large dark blue cuffs on its off-white coats. To a certain extent the functions required of a given group of soldiers were reflected in their dress. Thus artillery uniforms in most armies were usually of dark blue, for the practical reason that handling black powder would have soiled lighter coloured clothing. Infantry drummers and cavalry trumpeters often had "reverse" colours with coats the colour of the regimental facings and facings the colour of the regimental coats.
Officers (who paid for their own clothing) were slower to accept uniforms. During the late 17th century they were often dressed in individual styles and colours according to their own taste and means. In part this was because the uniform dress issued to the rank and file was considered a form of livery — the mark of a servant and demeaning to members of the social class from which officers came. One early practice in the French and other armies was for officers to wear coats of the facing colour of their regiments. Rank insignia as such was unknown until well into the 18th century. The gorget hanging from a chain around the neck (a last survival of medieval armour) was the only universally recognised mark of an officer until epaulettes developed from clusters of ribbons formerly worn on the shoulder. In the British army officers were ordered to adopt epaulettes by a clothing warrant dated 1768. Even when officers' uniforms became the subject of detailed regulation they remained easily distinguishable from those of other ranks, by the better quality and richness of the materials and trimmings used. Gold or silver braiding on the hats and coats of officers usually matched the bronze or pewter of the numerous buttons on regimental clothing.
New uniforms were issued with surprising frequency in some 18th-century armies (once a year in the British service). It should, however, be remembered that a soldier had to march, parade, fight and sometimes sleep in the same garment and that such extras as greatcoats or working clothes were seldom issued until the end of the century.

Ottoman Empire: The highly organised armies of the Ottoman Empire employed distinctive features of dress to distinguish one corps or class of soldier from another. An example would be the conical black hats of felt worn by the Deli cavalry of the early 19th century. However the basic costume was usually that of the tribal group or social class from which a particular class of warrior was drawn. As such it was sufficiently varied not to rank as "uniform" in the later sense. An elaborate system of colourful standards largely provided unit identification. Even the appearance of the Janissaries was likely to reflect individual means and taste, although red was a favoured colour and the white felt zarcola headdresses were similar.
It was not until the reorganisation of the Ottoman Army by Sultan Mahmud II during the 1820s that completely standardised dress was issued.

Later modern era: The first fifteen years of the 19th century influenced the appearance of military uniforms until the 1850s. In particular, some uniforms of the Grande Armée  – notably those of the cavalry regiments of the Imperial Guard – are considered as being amongst the most striking and distinctive of the time. The cost of the French uniforms varied widely, going from 200 to 250 francs for a line infantryman's outfit to 2000 francs for a cuirassier's uniform. Cavalrymen of the Guard had no fewer than 10 different uniforms. One justification for the expensive parade dresses of the Guard was that they would "lead the people of the conquered nations to regard the French uniforms with unreserved astonishment". As a general trend France and other European states replaced their bicornes by feathered shakos or crested helmets, while the coatee-type jacket replaced the combination of tailcoat and waistcoat.
The ornamental peak of the military uniform was reached in the early 19th century in Western Europe. Sometimes the Napoleonic Wars are identified as being the acme of colourful and ornate uniforms, but actually the several decades of relative peace that followed were a time of even more decorative styles and embellishments. The Napoleonic soldier on campaign was likely to present a shabby and nondescript appearance as unsuitable peacetime dress quickly deteriorated or was replaced with whatever local substitutes were available.

Until later on in the century dyes were primitive and different batches of uniforms worn by the same unit might present differing shades, especially after exposure to rain and sun. The white uniforms popular amongst many armies through the 18th and early 19th centuries soiled easily and had to be pipeclayed to retain any semblance of cleanliness. Green as worn by Jäger and Rifle regiments proved particularly prone to fading until suitable chemical dyes were devised in the 1890s.  British soldiers were known for their striking red clothing (hence the name "Redcoats"). This was actually a fairly dull shade of madder red until the general adoption of scarlet for tunics in the 1870s.

Navies: In an early instance of military camouflage, the sailors of Imperial Rome are reported to have worn blue/grey tunics.
However uniform dress was not a feature of navies (officers and marines excepted) until comparatively recent times. This may reflect the considerable difference in roles and conditions of service between sailors and soldiers.
No uniform was prescribed for the French Navy until a standardised suit for officers of blue and red was decreed in 1764. This evolved from optional blue clothing authorised by Royal decree as early as 1665, although practical grey, brown or black colours were worn at sea. In other 17th-18th navies of Mediterranean states red was a commonly worn colour.

Until the middle of the 19th century only officers and warrant officers in the Royal Navy wore regulated uniforms. Through the 18th century to the Napoleonic Wars navy officers had a form of dress broadly resembling that of army officers, though in dark blue with white facings. In the early 19th century Royal Navy officers developed a more distinctive form of uniform comprising (in full dress uniform) a cocked hat, dark blue coatee with white collar and cuffs, dark blue or white trousers, or breeches.
Epaulettes and braiding were gold and varied according to rank. In a simplified form this dress (without the cocked hat) survives as the modern ceremonial dress for flag officers.
Throughout this period sailors supplied or made their own clothing. Sailors developed traditional clothing suitable for their work: loose-fitting trousers with belts made of rope; tunics that slipped over the head, with arms to above the wrist so that the cloth would not foul in ropes passing through a cleat or pulley. For cold weather, a jumper was knitted from yarn or wool. For wet weather, old sail cloth was made into a coat (with hat or attached hood) that was waterproofed with tallow or fat. In these days, the officers would designate certain afternoons to "make and mend" (clothing). A sailor with little clothing to make or mend used this time as "time off".
In January 1857 the decision was taken to issue complete uniforms to petty officers and seamen.
This included features which can still be recognised in the Class I uniform of ratings in the modern Royal Navy - notably the wide blue collar with white tapes, a black neckerchief, white lanyard and blue or white jumper. The flared "bell bottom" trousers disappeared after the Second World War.
Because of the global dominance of the Royal Navy from Trafalgar to the Second World War RN uniforms became the model for virtually all other navies. While certain distinctive features emerged - such as the red pompon worn on the crown of the French sailor's cap, the open fronted jacket of the German Navy or the white round cap of the U.S. Navy - the overall pattern remained standard until the development of specialist working or protective rigs during the Second World War.

American Civil War: It is generally supposed that Union soldiers wore blue uniforms and Confederate soldiers wore grey ones.  However, this was only a generalisation.  Both the Union and the Confederacy drew up uniform regulations, but as a matter of practical reality neither side was able to fully equip its men at the outbreak of the war. Existing state units and quickly raised volunteer regiments on both sides wore a wide variety of styles and colours in the early stages of the war.  Some regiments—such as the North's Berdan Sharpshooters and the South's Alexandria Rifles—had green uniforms, while the French zouave style was widely imitated.