The electrophilic reactivity surface is also based on the total electron density surface.

"Electrophilic" means "electron-loving", thus the colors describe where new  bonding electrons will be placed.

In this case, what is "painted" onto the density surface is the value of the LUMO, which is the MO that accepts electrons.

Where the LUMO extends out past the surface, yellow, green and blue colors appear. Blue  is where OH^- most likely will bond.