Yellow is for non-emergency vehicles that require extra visibility.

When the NLEX was being upgraded, vehicles entering the contruction site were required to have them on. Yellow is also used for vehicles that are to be driven on active runways and taxiways in our airports.

Purple is for funeral processions but I think a lot of funeral services still don't know this since they keep on using red and blue lights on their hearses.

AFAIK, it is not illegal to own yellow.