Color Mixer is easy to use, but has always been a sort of blunt instrument because it lacked enough control in several ways. I don’t work for Adobe so I don’t know technically why Color Mixer is not part of masking, but the other thing I do know is that the new Point Color feature is much better and more controllable, so I now prefer Point Color and I rarely need Color Mixer.
It looks complicated, but if you want something simpler like Color Mixer and want to work fast, you can focus on the Hue, Sat, and Lum options and ignore the rest…just two steps, click to sample and drag a slider. But if you need to solve a color cast that Color Mixer was incapable of solving, then you can add more Point Color samplers, and expand Range and fine-tune all the range selections, as needed.

Point Color was very warmly welcomed by the pros, and I think it is one of the most important upgrades ever added to Camera Raw. If you want a good tutorial, try the one below by Blake Rudis, who is one of the more knowledgeable Camera Raw color experts out there because his background includes a solid grounding in color theory. (The Point Color section starts at 3:22, in case the link below does not already jump to that time.)
This tutorial by Colin Smith is very good too: