I agree it’s a cool and novel idea for a WordPress plugin and I do think you’ll have to continue moderating comments because like a few of us have said, it’s not a terribly effective CAPTCHA. Not to mention when you say “please make a bot that does this, we’d love to see it,” you can bet at least one or two people will do just that (if for no other reason because you said you’d love to see it, you know, to please you)

A common theme I think you’ll see is that people will say it’ll make it hard for color blind people to use it, one enhancement you may want to consider is actually putting the number on each color bar at a random location (this may even cut down on some of the stupider bots that may or may not be created.)