There are 2 ways you can do this depending on how complex your emitter is. The first way is to make the particle type color gradients the first color you want, and the tint color the second. Then animate the “tint strength” when you want to the tint color to take over.
The other way is to use 2 different emitters. Step the number graph of the first one down to 0 to turn it off and add the 2nd emitter at the frame you want the color to change.
You may also be able to use the “use next color” option (in the colors page of the properties dialog), but that would take some trial and error to figure out the value to set it to.
Alan.
http://www.wondertouch.com