"Quick - Think of a colour and a tool..."
It seems pretty unanimous that the reason this "works" is because Red is usually the color people think of first, and Hammer is the first too. It also seems pretty unanimous that nowhere near 98% of people answer "red hammer".
This page has a discussion from a linguist.
This blog post was also interesting.
My personal theory is that additionally once Red is in the mind, some people will then jump to Hammer easily because of the Russian Hammer and Sickle (and who thinks of a Sickle?)
The best page of responses was this one: