Color shifting reaction in black sealcoat
It's not a chemical reaction, AFAIK, but due to the Tyndall effect or Rayleigh scattering from the fine particles of the asphalt-water emulsion as water evaporates and the size and/or dispersion of the asphalt particles change.

It is also possible that interference of light in an adhered water layer plays some part. This is the same effect that gives soap bubble their rainbow colors, which change as the bubbles dry -- note that just before it pops from drying, one area of a bubble  may go colorless due to thinning of the film beyond the range of visible light interference.