The green colour is characteristic of TEB (Triethylborane) combustion, which is used in combination with TEA (Triethylaluminum) to ignite the Merlin 1D engines (those two substances are pyrophoric, and are used to kickstart the RP1/Oxygen combustion).
At least three of the nine engines are equipped with an onboard supply of TEA-TEB for in-flight relights. During the processing of the CRS-8 core in Port Canaveral, the remains of those reserves were dumped on the ground to safe the stage for transport. It's possible the first stages are now commanded to dump as much as possible as soon as the landing is confirmed, to reduce the risks & time needed once the stage is brought back to land (some was still left in JCSAT's first stage though, we could see the green flashes during Port Canaveral processing)