Gabriella Nanci has a good article on her website about Dexter Colors.
Also, on the UK Discussion Board Duncan MacIntyre makes a valiant effort in trying to explain simply the various genetic combinations with Color Inheritance.
Several years ago I was at a cattle show and I don't remember the cattle breed but I commented to the breeder about his dun cow. He kind of looked at me like I was crazy because as it turned out with his breed, although having the same appearance as a Dexter dun, their breed of cattle were considered either black or red. Red being the same as brown much as it was originally with Dexters. I believe that it was in 1978 that the UK made the distinction between dun and red as being separate colors and then the states followed about 10 years later. It took another 10 years for when color was added to the breed publication in 1998. Soon afterwards photos on the internet started to become more popular. Early on, unless they had happened to come across the colors in person or knew the color background of a few lines a lot of breeders weren't sure whether their Dexters were red or dun. Many of what had been originally registered as red were really dun and so /dun was added to the registrations. Now there's more live colored Dexters to view and pictures of red and dun Dexters as well as testing available to determine the color. If you're old enough to remember black and white television then you'll have some idea of the difficulty of differentiating between colors in the past.