1 if you are talking about the guards on duty it’s the colour of their uniforms and they don’t have any alternative colours for that particular duty. 2 it is the uniform that they wear while standing their turn as sentry and they are given regular changes of guard to account for the weather conditions and the heat of their bodies is not as great as it could be because they are not building their own body heat by remaining as still as possible. Once they return to the stables they will remove the heavy outerwear and the helmet and chest armour and put on their standard uniforms before returning to their horses, to give them a full grooming, after which the troopers are required to clean and inspect all of their horses tack. They also have to find the time to clean their horses stalls and give them fresh straw for bedding and they also have to clean and polish their own uniform and find time to eat their meals and undertake their own training through classes in all the activities that a member of the armed forces are required to undertake.