The yellow ribbon is already trademarked of sorts: it's strongly associated with bringing people home. I'd even argue that it's a specific shade of pale yellow. Taxicab yellow ribbons would feel off.

I'd argue that not all colors can be trademarked, but colors that strongly associated with their respective brands should be. They're not being taken entirely off the domain, they can be used in other logos, just not with the other color part of the trademark.

Here's a better example, Maize and Blue. Do you think you can identify UM by those two colors, what about those specific shades of yellow and blue? Now what if Ohio State wanted to switch school colors to those specific maize and blue colors, even with an entirely different logo. What if, they decide that when their football team is doing well, they'll wear the scarlet and grey and when they're losing they'll wear maize and blue (not that I'd mind :P). Should UM have the right to protect their use of that color combination?