after OnPlayerSpawn, set the Hitmen-Team's color to a value with 00 at the end, like 0xffffff00.
that 0x rr gg bb AA (red green blue ALPHA) values are important for you, have a look @ https://sampwiki.blast.hk/wiki/SetPlayerColor
it even works when u "change your skill"...
you could even set the color (black=hitman) to 0x000000ff for a black blip (visible) for a hitman "off-duty", and 0x00000000 for a full transparent (invisible) blip for being "on-duty"...