Not a must have. But useful. Especially because the AIME monsters are mostly stated out in it. Not all of them though... some are in premade campaigns and adventures, some are in the region books.
I don't think i've opened it during a session for any reason except for monster stats. Important to have, but you could get by with the free 5e open source stats or a 5e monster manual if you have one, and just tweek the monsters a bit for flavor and balance as you learn your party's capabilities.
Dang I never thought about that with these maps but I bet it must be pretty frustrating. I'm not sure how well it works, but have you tried using an app like this one? https://play.google.com/store/apps/details?id=com.colorblindpal.colorblindpal