This only applies to the colour varients. Pig and the slimes are not relevent to this.
I was on the DMU Store, buying the new hats. I scrolled all the way down, and saw the coloured fezes. This made me question something. The following image is from the DMU Public server, from NEI.
(it cycles through all wool colours in all slots, showing that off). You dye it by putting the fez with a dye . All dye bar bonemeal).
So why are the fezes on the store? Honestly, I am just curious why. It confuses me.