I have thought about using the same dye-sub method that some of the Spider-Man costumers are using. That way you can alternate between the red and gold undersuit. Sewing additional panels of different colored fabric is a great idea and looks great on the kids suit!
I also thought about using some piping in red, gold and silver to look like wiring and servos. The key is to have some sewing skills or better yet to have a wife with sewing skills!
This is one of the areas that most builders skimp out on and after a long build are just too tired to sink that much detail into that part of the costume. Hopefully we can pool our knowledge and get an undersuit worthy of some of the outstanding armors out there!