The yellow boarding passes came out of the kiosks. The purple/pink ones went to elites at counter checkins. Recently at the YYZ T2 domestic SE/Elite/STARGold check-in, I have received white cards with a pale blue "watermarked" logo, most attractive of the bunch.

The yellow kiosk ones were designed to alert gate agents to check for ticket coupons, upgrade certificates and other items which check-in agents might otherwise insert into the pocket for pulling upon boarding.

UA was one of the pioneers issuing these types of colour coded boarding passes, reserving Gold ones for their elites and F and C customers.