I doubt it has anything to do with repair. When you’ve got a 18 or so layer PCB, probably with microvias (invisible if they’re in pads), and very closely spaced

I doubt it has anything to do with repair. When you’ve got a 18 or so layer PCB, probably with microvias (invisible if they’re in pads), and very closely spaced QFN and BGA-packaged parts everywhere, soldermask colour is the least of your problems. So much of the design is on internal layers inside the board and so much interconnection to internal layers in pads (if using microvias) and under BGAs that being able to easily see the top and bottom layer traces easily would give you very little to go on. You’d really need design files to do anything more than just reflowing the odd part... It probably is a lot to do with fashion though. It’s literally one of the only reasons that I always specify blue or black soldermask at work - just looks a lot cooler. Green boards just have that late-90’s electronics look.