Different colors depending on the panels. Grey for white, black for grey or other dark colors, brown for yellow/orange/red. Panel lines are shadows for the panels, so it makes sense to use a darker color than the color being shadowed instead of one color all over. It is, however, a preference, you can use what we color you want, and black all over gives an anime look. It depends on how much sanding you do and how large the sanding material is. There are too many variables to give a definite answer.
So my bf is the one who has gotten really into building these kits, he has built HG, MG, and one Perfect Grade. I want to get him his first RG for his birthday coming up. There are three main ones I’m looking at the Nu Gundam Char’s Counterattack and then MK-II titans Z and the RG Unicorn but since I don’t know much about them is one any better of a build than the others?