When I flew Grand Canyon tours, not for Scenic though, we did that so we could identify which passengers were for each airplane. Most of the passengers did not speak English so we'd hold up their color sticker so they'd know which pilot to follow and we'd look for the stickers to know which passengers were ours. The stickers were also numbered to match the passenger names on our manifest. Really helped when dealing with a large group of passengers when there is no common language.