The spectra of Mars material can be estimated from the SNC meteorites. When that material, which was ejected by hyper velocity impact from Mars, collided with Phobos and Deimos, it would not have covered them entirely, but rather combined with what they are made of in low speed impacts. The mixed debris would then re-accrete.

The quetion is, is it possible to estimate the original composition of Phobos and Deimos by subtracting the spectra of Mars material from the currently observed spectra of Phobos and Deimos?

Obtaining cores from Phobos and Deimos will undoubtedly be among the goals of the first manned flights to Mars orbit.