I may have missed something but I don't know if we have a specific outline mapped out in source material. Someone else more knowledgeable could chime in about it.
Personally, you can't go wrong with just standard legion colors. I would imagine that it wouldn't be parade standard or even battle standard colors. I would even go so far as to say that it wouldn't match much in the way of legion colors to help offset the dishonour the beast would be on the parent chapter. Perhaps even give it proto-legion colors, suitably marred and weathered as to home in on the psychotic nature of the creature. The great thing about not having source materiel is that it frees you to do whatever you think would be best for the narrative of your army.