What happened when I painted white and the color turned yellow? Urgent help!!!

It's a chemical reaction between two different paints. Let's say you use an alkyd type of paint for the white paint and a nitro type for the outer layer of varnish, you'll have the problem you're talking about. It will also blister and peel later. To remedy the problem, you have to "bite off" the original paint layer, that is, you need to use a lot of asphaltene to clean off the paint, and then use the same kind of paint construction.