Is the stain on the inside or outside? If it's on the inside then you're probably okay b/c you can turn it inside out and protect the outside. I know that sun tends to bleach things and it may or may not bleach your wool. Hopefully someone else knows more about it than me!
It is on the inside. And since its a natural color it wouldnt matter if it got bleached -- I mean, I guess that is what I want, for the sun to bleach the yellow out. But I wonder if it would make it worse?