I think it's perfectly acceptable to go over the "colour limit" imposed here. Colour restrictions really help unify work, force you to incorporate hues into areas you would normally not, and generally tighten the piece so it doesn't seem awash in colour. It is also necessary at times when you are restricted by technology or medium. Frankly, the piece is finished, so if you want to add another set of colours to add another element, I don't see any issue with that... but I guess I would ask, what colours are in that border that you couldn't pull from somewhere else in the picture to keep the same colour limit if it is important to you?