The First Foot Guards
We are a Revolutionary War
reenactment group based in Boston MA,
accurately portraying the royal household regiment that is now known as
The Grenadier Guards
You can blame Brigadier Harry Lumsden for khaki.
The word 'khaki' comes from the Urdu word that means 'dusty' or 'dust-colored'. Its history is connected with the British Army's residence in India and Afghanistan in the 19th century.
In 1848 Brigadier General Sir Harry Burnett Lumsden, in equipping his army corps against the Afghans, had the uniforms dyed khaki color. Gradually, all uniform cloths dyed such a shade were called khaki, although the fabrics included drill, serge, and whipcord. Drill, serge, and whipcord are different weaves of various cloths, a common characteristic of them all being strength.
The British National Army Museum website has a portrait of Harry Lumsden (Go to "Collections" and then do a keyword search for khaki.) The site will give you further details of Lumsden and his regiment, the Queen's Own Corps of Guides.
The last campaign use of the fabled red coat came in the Anglo-Boer War (1890s-1900s). Clearly the Boers, who conducted a largely guerilla war, found the red coats an easy target.
The Columbia Encyclopedia defines khaki as a "closely twilled cloth of linen or cotton, dyed a dust color." The color changes came with World War I. Fairchild's Dictionary of Textiles says that "after the US entered World War I, an olive tint was added to the color to make the khaki invisible both against the bare ground and the foliage.
Nowadays 'khaki' refers to both the color and the material. Either way it is a much more practical color for a modern day combatant. In the 1700s, however, different tactics were used, and a serried row of redcoats approaching at slow march with bayonets fixed, and the intention of using them, must have been an awesome deterrent.