Wanna bet?
To be fair, the original Ultra Magnus toy from G1 - being a repaint of Diaclone Powered Convoy, was basically a white Optimus Prime but with a different trailer that happened to transform into a suit of armour for him. While that armour has undoubtedly become an iconic part of Ultra Magnus' image the fact is that the true robot form
is a white repaint of Optimus Prime (no doubt thanks to the fact that G1 artists
always drew him inside his armour and we never saw him in his basic robot form).
I really liked how Dreamwave showed Ultra Magnus in his true core robot form (after having his external armour stripped away).
Having said that, it would've been nice if they could incorporate the trailer colours with the cab robot as it would break up the monotonous white and make it look nicer. We've already seen Takara do something like that with Laser Ultra Magnus and it'll be interesting to see if TakaraTOMY will do something along those lines with Henkei! Henkei! Ultra Magnus (presuming that UM will come out in Henkei). So while I agree that doing a white repaint of Prime as Ultra Magnus all the time is very much minimal in effort and rather bland, it is inaccurate to say that Ultra Magnus "shouldn't be" a white Prime repaint - because quite frankly, it
is G1 accurate! But in the absence of a trailer/armour it is bland (if you only had the G1 Ultra Magnus cab robot with no trailer it would suck as a toy).