I really like what you did here in highlighting certain aspects of the undersuit.
I was planning on hand drawing the layers I was thinking of for my reach undersuit, but it probably is a lot easier to just edit a photo like you have
I would say that the color at the top/front of the chest for a REACH build would be a layer of foam between the undersuit and the actual armor pieces. In Reach, there are sections where there is like an upper and lower undersuit, like the interior of the thighs for example.
As for this Halo 3 Master Chief image, i would say that you could get away with it all being a single level of undersuit rather than having layers of undersuit.
Lastly, in regards to the red on the back of the helmet: Yes, that is part of the helmet, but some people in the past have made that area removable or out of rubber so that they can take the helmet off or put it on more easily.