\x06 is just the 'custom' colour in the OBV engine, it's specified at runtime in the client code by CBaseHudChat::SetCustomColor(const char *).
For example TF2 uses it for the item found messages, where it sets it to the colour representing the quality of the found item.
It's black before it is set in code.
Maybe it could be added with a {custom} name, just please not {black}.