Yes, more headaches, and again it's the Van Konijnenburg set I'm bothered about! This here is a colour proof of the 15c ultramarine. Or, well, I hope so anyway.
Now normally you can tell most colour proofs apart from the real stuff because they're either imperforate or they're in a colour which was not chosen. The problem with this example here is that it is in the chosen colour and on top of that, the fact that it is imperforate is no guarantee either. Apparently, stocks of unfinished stamps (printed but not yet perforated or gummed) haven fallen into the wrong hands and have ended up on the philatelic market.
Now that's all very fine but does that mean that there is absolutely no way of telling which is which? If you happen to know, enlighten me, please! Until then, I'll just mount it with a proviso.