Heyo! Yellow is one of the lightest hues on the color wheel, so this is a very common struggle. What I do for this is I choose my yellow first. I make sure it is pale but still saturated a bit. Like this, and this color.

You should use white in moderation, usually only for highlights. Even then, I would recommend keeping it still tinted yellow. For the shadows, tint it to a peachy orange more like this. I think you might be choosing yellows that aren’t saturated enough, so they blend with the white.

i hope this helps! I know it can be very confusing aha
(Also great job on the creeper mage!)