There are things they simply can't go back on. This would be flying all over again.
Making most items blue, and rare procs purple will only serve the purpose of making players (who care about the color anyway) feel less powerful even if they're not. Plus it rewards luck on RNG and nothing more. Item color is not associated so much with rarity as it is with quality. It doesn't mean so much "this item has a very low drop rate" as it means "this item is not as easy to obtain".
Even if you can make every player understand that the color doesnt matter, stats do, and the color only means how rare it is, it will still mean mostly nothing.
The most they can do, is simply replacing the mythic tag with a new Mythic rarity with a different item color.
If they're feeling frisky, they might make most of LFR drops blue, so you'd have blues from LFR and HC dungeons, epics from Mythic dungeons and Normal/Heroic Raiding, and mythics from Mythic raiding.
^ If you want to show of about RNG drops you already have some gear and mounts. Otherwise you have Mythic gear.