Medium blue (like a lot of the current g* discs) is the best for me hands down as I have some colorblindness.
Other good ones
White, bright yellow, bright orange, some purples
I cannot see well: any greens, any pinks, dark orange, dark yellow, and tiedye. Red is very hard, almost impossible to find for me.
Answering two other questions, for me I can't see red easily on green grass, but I can tell a red disc from other discs when looking at them in my hands, the difference is subtle.
And yes knowing the disc color you are searching for helps me alot because I also look for flashes of color rather than shape. If the thrown color is red I also help look, you never know I might trip on it.