Why can't I use solid colors on certain cosmetics in Fortnite?

If you receive an error message while adjusting the color of your cosmetic to solid black or white, this is intentional for certain cosmetics since it may conflict with the primary color. 

To check if the color set is blocked, select the desired color. You will see a message along with a blocked symbol over the chosen color.

In this case, select a different color that's not blocked by the system.