Given the new graphic, I'm really confused why they don't just print on the boarding pass and verbally say Zones 1-8 (I think that's how AA does it?), in addition to the verbal name. i.e., "Now boarding Zone 3, Delta Comfort+."
I get the idea that the color on a mobile boarding pass could match the color on the gate area screens as a nice secondary indicator that you're boarding at the right time, but a straight set of numbers seems easiest for people (especially non-frequent fliers) could understand.
I'd also wager that color BPs are the minority. I can't imagine them upgrading to color printers at kiosk or agent machines. I only have a B&W laser printer at home, and many people with color probably still print in B&W to save on toner.
GIVE US NUMBERS, DELTA! It's okay to admit that AA does something right when it's actually true! Don't let ego and "Zone 1" Amex advertisements stand in the way of real progress!