The flashback-light

MentalMill v1.1 was released recently and I played with it, trying to solve a
puzzle one of the moderators provided:

When light with the right ‘wavelength’ hits the surface, suddenly
some patterns show up (similar to the typical blacklight effect).

So I came up with the flashback-light, a flashlight which reveals how
something looked like in the past.

Here is the video of 3DSMax’s DX10 viewport.
You can also download the Max2011 scene file and the related assets.

The shader contains two lighting calculations: the first one is a phong
function with a constant white diffuse color, which makes it easier to
separate the lighting of a specific color from the rest.

When the lighting based mask is generated, the final, lit surface can be
created, using all the textures blended by the mask.

I’d be quite interested in hearing about other solutions for this problem.