For whatever reasons, my K10D seemed to do reds better by far than my K5. If you are not very careful you will blow the reds with the K5 almost every time. Raw helps, but will not eliminate the problem in the majority of cases where I have blown reds. Other colors are fairly easily corrected, but red refuses to cooperate, even in Raw far too often.
I do agree,it is not just Pentax, I see it in almost every brand to some extent. I would like to see some Sigma examples....maybe they are better?
Regards!