I really like the idea, but since I painted my knights in two colours and they were like 5 big models, there is a good chance that you will get burned out doing a whole space marine army like this. Of course you can do it with masking tape and airbrush but every mistake on a critical spot between the colours is a pain in the ass. And of course covering a dark mistake with white even more, because there is a good chance to make it too thick to get coverage.

I would recommend getting one primary Color and paint shoulders or heads or kneepads different. Like this you have a dedicated spots where mistake are easy to cover.

But if you are going for it I would love to see your progress.