fractic: The trick is that black can play now. Normally this would serve little purpose.
fractic: After and white can't play at a because of the strange ko rule. Now it get's a little tricky. If white plays anywhere on the board (note white has no ko threats) black can play at a himself repeating the whole ordeal. So white will pass.
If passes don't lift the ko ban then black also passes and the result is jigo (40-40). If the pass does lift the ko ban then black plays a and the result is an endless repetition.
Under normal rules black can't prevent white from playing at a so he'll lose by one point.
fractic: Of course but those rules are part of their ko-rules. The way the colourblind ko rule is written on the parent page implies that passes don't lift the ko ban. Of course banning such repetitions would make sense for actual play. The problem was just intended to show that the colourblind ko rule could make a difference in the result of a game. (And I think black 3 is a nice move).
Bill: Yes, it is nice, and the problem is very clever, as well.
Also, the rule does imply that passes do not lift the ko ban. So why even mention the possibility? :)