This K-5 user has consistently complained about the colour inaccuracy and gloominess of the K-5 of his. He posted various comparison shots and tried to illustrate the problem:-
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=37834071
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=37840634
I won't "spoil" his results this time. Go read and judge yourself for the "issue".
As for the colour accuracy and response of my K-r, what I want to say is that I am satisfied. It is really good. That says it is yet slightly better than the K-x and my Cantax K5D Full Frame, obviously better than my Sony NEX. Enough said.
I don't know if the K-5 would be (much) different. But at least for those gloomy pictures that the user complains about should be an underexposure / exposure inaccuracy issue, instead of a colour inaccuracy problem, I believe.