Originally posted by Maffer I would say that the Sigma lens could be the culprit here. My Pentax kit lenses which came with the k200d seem to render every shot with more vivid colors than the Sigma ones I own (10-20mm & 70mm macro) using the exactly same settings on the camera. However it's not a showstopper for me and can be dealt with post processing if saturation is really needed.
After 2 months owning the Pentax, I concur. The lens makes a huge difference in saturation and in general picture quality.
I love my K200D with my old SMC Pentax 1.4 manual lens and my super takumar 50 1.8
and all the other manuals i own.
My kit lens....meh, it is just not as fun as the old school manuals.