When an electron returns to it's ground state does it release the same wavelength of light that promoted it in the first place?
Not necessarily... it can return to the ground state in multiple steps, releasing different wavelengths each time.

Also, the excitation could be caused by something other than light in the first place (heat, for example).

Also, relaxation isn't instantaneous, so the emitted photon wouldn't really be "cancelling out" the color of the absorbed one as some time has passed.