"Best colour" for tanking mages?
Arguably colourless, but prior to AR I would say that green was probably the best colour because red mages were uncommon. Especially before the Ragnarok refine. And W.Tharja has no speed, so without bold fighter her player phase was really weak and a green with high res could at least bait her in for someone to mop up.
Now that Ragnarok has that ridiculous refine and bolt traps hit both ways and Surtr has provoked a bunch of Lilina and red Micaiah, maybe blue is the best colour for tanking mages in AR? I feel like I see more of Myrrh and Surtr than I do of green mages.
Before weapon refinery and Winter Tharja, blue was the worst color for magic tanking due to red not being much prevalent. Especially against lances, Reinhardt is still a force due to the ability to hold Lancebreaker, so that puts lances off his counter list.
Right now things have changed for me. AR has forced me to use Nailah as one of my ranged counters, and I also use Deflect Magic to screw some Reinhardts.