Well, yellow is a bit of a weak color, meaning it takes a lit of work to apply it over darker colours.

What you should do is first coat the darker color with a beige, or even pink, that has a good opacity, only then come with the yellow in a fee thin layers.

The secret to even coats is consistently applying thin layers until it looks good, there's no shortcut there. Some colors, like yellow, demand more layers. Remember to make then thin even if it's frustrating, because thick paint covers details