If I understand you correctly, (and i hope I do), you want basically want to be able to drive where color's position on the ramp will be?

to test I set up a 3 color circle ramp on a nurbs plane. The top color being white(colorEntryList[2]), and the bottom 2 black(colorEntryList[0] & [1]) . The I used the following expression to test it out.

ramp3.colorEntryList[2].position = nurbsPlane1.translateY (replace this with whatever)

as i moved my plane along the Y-axis, the circle grew and contracted, as a pupil would.

Hopefully this was what you meant!