Diffusion and coins

[mohemoto]

Hey guys, I'm not sure if this is the correct topic to post this under, and I apologize if it isn't. For an assignment in one of my classes, I'm asked to design a procedure that will allow me to change the colour of any type of coin through diffusion. The procedure can involve any variables, as long as there is a colour change, and diffusion causes it to occur. Could anybody suggest a way that this would be possible?

 

Thanks a lot!