This is the ink pad that I have
So, what I did was carefully position my stamp over the red, orange and yellow portion, then dab as I moved the stamp (which is a lot longer than the ink pad) along the ink pad, careful to keep the colors more or less lined up. Then I stamped those three colors before going back for the next three colors. Since there is no purple here, I took a separate cat eye chalk ink pad and applied that over the red ink that was on my stamp from the previously mentioned process. The result? This fabulous gradient rainbow!! I was pretty happy with how it turned out! I can't wait to think of some more stuff to do with this ink pad!