I get the occasional odd question about preshading on my Gundam kits/gunpla, so I guess I’ll put up a short guide with some references for you to look at.
First is this tutorial from Hobby Japan that someone translated ages ago. It’s about the “Max Technique” shading technique that modeller Max Watanabe popularised in Gunpla (though it’s something aircraft modellers have been doing for some time already).
This teaches you the basics of shading, but I find there are a few problems I find with this Max technique. For one it’s extremely time consuming, since you have to do several coats, add white, do several more coats etc, to get the full colour to come out. For example, to paint the white parts on my Strike Freedom/God Gundam, it took about 2 hours for me to paint the 2nd coat (white + navy blue), and then another 2-3 hours to paint the final white highlight layer. The reason why it takes so long is because without enough coats, the white looks gray instead.
I only use black undercoat for painting white as well. Other colors such as blue/red/yellow etc, don’t paint well over black and the colour is quite affected. For these colours, first I decide on the final colour I want. Add some black to this and call it your base layer. Then use your original colour minus the black and paint away from the edges. Finally add a bit of white and paint away from the edges again. This will give it a nice colour rise shading effect.
Also watch this video on YouTube of a JP guy painting a RX-78 KA. He does a nice job of shading (he just adds white for highlights). This also shows that you don’t have to start with a black coat for the best results.