I am convinced that colorblind people key in on the shape of the disc and so can often (but certainly not always) find lost discs better than normal visioned do. I find a much larger percentage of other people's than they find mine.
If I can make an educated guess, I would say that the colorblind people you've witnessed searching for discs were working much MUCH harder than you, that's all. Blending all sorts of shades of colors together doesn't make it easier to find anything And the shape doesn't stand out the way you're imagining. Find me a colorblind person who says they find discs easier because they can see the circle and I'll eat my words. When I'm looking for a disc I'm seriously looking like a madman because 1) I don't want to annoy the people I'm with, 2) I don't want to hold anyone up behind us, and 3) I don't want to lose another disc!