The three versions of the same painting by Warhol in three different colours reminded me that charge for the strong interaction comes in three types. Electromagnetic forces have two types of charge and these are called + and -. But there are three quarks needed to make a baryon and three together would add up to zero. The idea for colour charge came from the way that red, green and blue add up to make white in the primary colour definition. So the three quarks in a baryon would be assigned red, green and blue charges to make that baryon colourless.
