A dark transfer is fine to use on white shirts. "Dark" transfers really aren't dark - they are white.I think design will so up a lot brighter (IMO), but you may see the white transfer on the white shirt.
The other transfers are clear so all the white color in your design is actually clear - these are made to use on a white shirt, and the white of the shirt comes through the clear.
Does that make any sense? I like to use the dark transfers no matter the color of the shirt. I think they hold up better after washing and the colors come out better.
Hope this helps!
*If I have said anything incorrect someone please correct me!*