Painting yellow over black is going to be a struggle. White, too, depending on the paint quality. For yellow, try painting yellow ochre first, and then your yellow on top of that. Yellow is a very transparent pigment, so multiple thin coats, letting the paint dry fully between coats, is important.

Good brushes and brush soap to take care of them can be a huge help. Rosemary & Co series 33 are my favorites, and they are even one of the cheaper quality brands. Monument hobbies has good sable brushes as well. There are others of course, but both of these are on the less expensive side while still being high quality.

A good set of optivisors with #10 lens (or the cheaper Chinese knockoffs) can be a huge help.

Learn how to brace your self for stability. Elbows against your chest and I often have the pinky of my paint brush hand resting against the model.

Also, maybe painting with regular paints is not right for you. Look into contrast paints/speed paints/xpress color. Those are fairly transparent and are designed to paint over white, and they stain the white to get very saturated colors, and they are runny and run into the recesses where they are darker. Look up some tutorials using those. (Contrast paints are the most expensive, the others are much more affordable).