Don't use Vallejo, but traditionally yellow is a very "thin" paint, and the undercoat shows through. SO choose a lightish, neutral base - I've been using light grey, but will experiment with flesh paint next time I do yellow. THen several coats, trying to minimise brushstrokes showing.
Another alternative, which worked well with my Dig Lord - base coat in metal. Then apply the yellow. Don't be too fussed by the brushwork showing. Do a few coats. Leave metallic bits exposed. Then hit with black wash. It's meant to look slightly dinged and dirty, no?