Painting yellow.

What is a good wash to use for yellow?

Depends on what kind of effect your trying to get.

For weathering, I'd try chestnut.

For grossness, I'd use green ink

I agree chestnut is nice...flesh wash works, as well -- very similar to chestnut, though.

I occasionally use brown, as well.

Yellow is tricky. I'd consider using one of the darker citadel yellows as a base and building up from that, instead of using an ink.

I tend to use bubonic brown as a base and build it up from there

I usually use a mix of yellow+orange ink mixed with Future floor cleaner and a dab of water to thin it down. Bill you remember my Autarch on the jet bike? He was painted using that method.