I made a proof-of-concept implementation and, well, it works, but I suspect they're too difficult for many people. They can be made easier by emphasizing one of the colors in the letters, but that opens them up to more traditional attacks.

http://www.anaerob.dk/animcap/k,w.gif

http://www.anaerob.dk/animcap/r,g,b,c,m,w.gif

http://www.anaerob.dk/animcap/r,g,b,c,m,y,k,w.gif

http://www.anaerob.dk/animcap/r,g,b,c,m,y,w.gif

http://www.anaerob.dk/animcap/r,g,b,c,m,y.gif

http://www.anaerob.dk/animcap/r,g,b,c,m.gif

http://www.anaerob.dk/animcap/r,g,b,m,y.gif

http://www.anaerob.dk/animcap/r,g,b,m.gif

http://www.anaerob.dk/animcap/r,g,b,y,k,w.gif

http://www.anaerob.dk/animcap/r,g,b,y,w.gif

http://www.anaerob.dk/animcap/r,g,b,y.gif

http://www.anaerob.dk/animcap/r,g,b.gif