Method one-- scoop out a bit of the soap after the cook, stir in color, then glop it back into the crock. Mix it once or twice, then pour. It will have a marbled appearance.
Here's an example:
Method two - make a divider in the crockpot with a piece of cardboard. Color each half (or third, or whatever you are doing), pull out the divider, and stir once or twice around with a spoon. Pour.