I'm pretty sure that reason for that is because of the size, since the miniDVD's are about 8cm in length compared to how DVD's are 12cm, so they defenitely didn't have as much space to work with on putting full color images and might have been worried about the color saturation when the images were resized to a smaller space. So they stuck with the lack of color and kept it simple, which depending on some games can look really cool( for most of the gamecube games). However, on the Wii it doesn't really seem very necessary and just doesn't look as good, considering comparsions to other platform discs' colors.