Many people are familiar with thermochromatic and light-sensitive inks that change color based on changes in temperature or light. Coors beer cans, "mood" rings, and color-changing nail polish are examples.

What I am looking for is a way to (invisibly) treat paper so that the ink from a single pen will look different depending on whether the ink is used on untreated paper or treated paper. For example, writing on the untreated paper, the ink will appear blue, while writing on the treated paper (with the same pen) the ink will appear red.

I have several uses for this that I would be happy to share if someone could point me in the right direction. There used to be (or still are) markers that write in one color, but when wiped with a clear substance, the ink would change color.

In essence, I'm looking for a pen that will write in two different colors without having to fiddle with the pen.

Thanks,
DK