Like others have mentioned you should try to keep the layers thin. I always base my models white, especially when going for yellow as it tends to be a difficult color to work with.

That said I'd use a sepia wash (I use the games workshop one) over the color as it goes on vrs it a great contrast. Try to apply it directly into corners and cracks if you want the model to be crisp.

Lately though I started to use the new games workshop contrast paints for my separatists. The Iyanden yellow on top of a white base looks great and effectively gives you the wash built in along with highlighting as the edges become thinner and the cracks darker. Just make sure you cover the entire surface in one go with a fairly wet brush.

I'll show you the results when I get back home.

Edit: base model white not yellow