That's a subset of the objective. The objective is to detect all humans because they wouldn't spot the 'face' in the coloured circles. Whether or not it's coloured doesn't change that.
Unless you're concerned about the rights of colour-blind computers for some reason.
How do you know if the individual being tested doesn't see a face? You have to trust them when they say "no face here"... May as well just ask "are you human?"