So, I can see plenty of people have looked at this post but not answered which makes me think it could be a common issue. I’ve done a workaround which keeps me going for now.
Instead of a long thin rectangle, I’ve produce a square that is transparent with the same rectangle in the centre. I’ve then made a slider that is completely transparent apart from the square knob. Underneath this slider I’ve then added an image which is my colour band. This works exactly how I’d like but I’m sure there must be a cleaner solution?