Changing Color Ramp of SKEmitterNode in code

The problem I have, is that I cannot figure out how to change the color of a particular item (keyframe) of the color ramp. In the spritekit particle file, I create a color ramp with multiple colors at different times. Now, I want to change one of those colors and keep the time. I think "setKeyFrame()" is the right method to use, but the color does not seem to change. Any explanation on how I can make this work?