For school socks I like to stick with a solid, it creates a sharp image. My girls prefer bright colors, though I actually think black or grey goes best with their uniform and looks the most sophisticated. We alternate, I bought a couple of Silky Toes 3-packs in different shades and they switch between the different colors.
Just love the look! Definitely trying those...
I do believe kids should be allowed more self expression. If your children prefer colored socks, that is their way of expressing themselves. Perhaps always do solid colors.