The best would probably be to look through the viewfinder with different lenses on as colour differences between lenses are a lot more subtle than those from post processing. That said when you review a portfolio of pictures taken with mixed lenses you can see patterns in how they render images.

Personally I don't really have a preference between Tamron and current Pentax rendering. I prefer both of these to (the admittedly cheap) Sigmas I have and prefer some of my older Pentax lenses to any of the new.