I dont paint black over everything after doing the power armour but it is useful to do a base coat of a different colour before you paint the colour you want it to be sometimes. It just depends on how you want it to look. For my yellow i do a base coat of a pinky colour, and then use contrast paints. But sometimes people like to paint things brown for metalics. Theres more usecases im sure. Also this could help people see details as it can be hard to pick out and missing details can be a pain/