Surely you can't trademark a colour? People have been copying the general liveries of Gulf and Martini etc cars for decades with no problems.
It is the combination of the two colours. I am sure if Charlie paid Gulf for sponsorship or asked them nicely they won't mind. Remember my Imp, almost modern Gulf colours but just a few hews different.
Usually the paint codes belong to paint companies so, as said above, it comes down to combinations.. not as if even the colour was Gulf's to begin with.. I think the line to not step over is using the Gulf logo but even then you'd think they'd welcome the unpaid advertising space.. corporate huh.?