A few thoughts from my own experiences:
Had a 2017 WRX STI in WRB. It's amazing color that shifts a little bit depending on the ambient light. Never had to worry about any color matching, the only downside to WRB is that it's bold. Everyone would know when I was at work because my car stuck out. Granted it was a lowered STI with a giant wing, so maybe WRB wasn't entirely to blame. It's a great color and unique to Subaru. I believe it's a tri-coat and looked really nice just before sunset.
I'm not sure how much lighter or darker MGM is vs. Subaru's previous Graphite Grey Pearl (GGP). It's an easy color to match, in fact, a friend's kid scratched my car with their bike (the horror!) and a body shop was able to touch up the scratches extremely well without repainting the panel, really easy to maintain and hid dirt pretty well too. I think the gray (or any of the colors) would really pop if you have the black trim pieces removed and sprayed in gloss black vs. the matte color. Had a sparkle to it, especially when freshly washed. I originally wanted black, but the dealer pulled some shenanigans and the next dealer over was straight shooter with a grey WRX in stock, ended up loving the color even though it wasn't on my radar initially. Grey with gloss black accents and bronze wheels looks pretty amazing IMO.
The Silver and Red are used a lot in the press materials and both look great.
I ordered CWP this time, although i'm still thinking I should have gotten silver, grey, or even red. The bottom line is they're all great and I don't see any dud colors. My plan with CWP is to wrap the roof black, tint the windows, and maybe throw on black wheels for a black/white theme.