To the OP - dude, you could drive yourself crazy playing this game. There's always variation, among the same model (different screen mfrs, etc), various models, etc. We've all done it, and most of the time I have to admit I wouldn't have noticed or considered most of the "issues" a problem except for the board comments.
The iPad Mini retina definitely displays colors differently, and is less saturated than the iPad Air. It has a smaller color gamut. Similarly, my MacBook Air 11" doesn't have the best screen in the MacBook lineup, not by a longshot. But the size and portability work great for me, and I love it so I choose not to antagonize myself by comparing it with larger/better screens on other models that look great don't meet my particular requirements as as well. If I need an accurate, color-calibrated display for editing photos, I go fire up my Mac Pro, because the iPad Mini isn't the tool for that stuff anyway.
If you like the size of the iPad Mini, like I do, then make a choice to keep it and appreciate it for what it is, a smaller, slightly more portable option than the iPad Air with a very nice display. Don't put it next to other models or try to find fault/prove to yourself that the screen isn't quite as good as those, because you will.
Hmmm... a lot of the above is true of just about anything in life, isn't it? (Sorry. I'll resist the urge to digress into an existential ramble here.
Bottom line - if the screen is a priority for you, and the other features of the iPad Mini aren't attractive enough to make you feel good about it and accept the whole package, you're always going to be disappointed. Take it back and get an iPad Air, or whatever makes you happy.