Soldiers in the Regular Army, (by which I mean, excluding those men who joined up or were conscripted for the duration of hostilities) were issued with a number of different types of uniform, mainly described using numbers. So No1 Dress is the most formal and would be worn on parade for ceremonial occasions. The next level, No 2 Undress, is for slightly less formal occasions, but was not designed for combat. There are then several categories of dress which relate to warmer climates which we need not go into here. And then there were the various orders of khaki service dress for combat and workwear within barracks, which others have already mentioned. The service dress that the KRRC wore during the First World War was pretty much identical to that worn by all other soldiers in a general infantry role. I assume you are not asking about that uniform.
So concentrating on No.1 and 2 Dress, the KRRC wore substantially the same uniform as other regiments in the Rifle Brigade, that is to say a black shako cap, rifle green jacket and trousers, (with black frogging on the No1 jacket) and a black stripe on the trouser leg, and black buttons. What distinguished the KRRC from other members of the Rifle Brigade, was the crimson collar and cuffs (facings) of the KRRC, and their Maltese Cross cap badge. You can see an example of the uniform dating from 1910
here. As far as I am aware this was unchanged ten years later. There is some artistic licence in that painting which implies there was a difference in colour between the tunic and the trousers. Army dress regulations from the time specify that they should both be rifle green.
