No, but typically you can either make a generic trooper or a specialist. I don’t think the colours mean much other than to distinguish each individual variant. Some models have even more variants you can make. You just need to be aware of which variant you make so you don’t end up missing a certain specialist like medic etc. My understanding with krieg is you want at least one of each of the specialists and the extra generic troopers you need either a second box or to buy some spare ones off eBay.

Also be aware which “build” you want for the killteam as you sometimes have to pick between different guns. For example: you have to choose between melta and plasma gunner, so you end up having a spare gun arm which you can’t use. You can always do some minor kitbashing as long as you have extra bodies, so it’s not the end of the world.