Color Ramp controlled by invisible objects position?

Hi!

First sorry if my English is dodgy. It's not my native language ;)

I have an idea and can't figure out, how I should implement it. There's quite a lot of tutorials about iridescent material s out there. They all are made using a color ramp. So the colors are on static positions on the object, unless you move the camera and/or the object. My idea would be to "move" the colors on the object, depending on the position of another (invisible) object. Can anybody tell me how to do this?

Thanks in advance!