I like the art style on the United cards. The lack of true black lining I think is what makes these semi-Chibi models really nice is that they have a more organic feel.
Download paintRack (it's on Android and iOS) and use their Color Tools to find a match in whatever brand you have access to. I can't possibly list all the possible matches here.
I'd suggest that you load up paintRack on iOS or Android and use the Color Match tool. Amethyst Rose is pretty close to Khorne Red, and Khador Red Base is comparable to Mephiston Red though. The rest I don't know off the top of my head.
