I don't use GW paints but all paints are similar... my lizards are also not dirty looking. I'll post up some pics later but they look pretty clean, and I use a lot of washes. The trick is to paint over it again for highlights or even a second coat of mid-tone after you apply the wash. I apply gloss finish afterwards to achieve that shiny, wet look that reptiles have.
It's a bit more time consuming but the result is worth it.