Colour prejudice?

The latest in the “interesting” questions thread:

Following, with interest,┬áthe discussion regarding differing Glen of Imaal Terrier breed standards something sprang to mind. What about the colour clause? Why are some colours acknowledged and some not? The “and tan” colour is seen in many countries and so is the liver so why isn’t it there in the standard? If working ability is all, and those are the colours of dogs with good instincts, are they worked rather than shown?