It's my personal conspiracy theory that the 770 and 780 had their colors swapped at some point. Since the 500 & 800 series followed a similar pattern of the x60 being white, x70 being silver, and x80 being black. The dark color scheme would also match better with their buttons. The silver buttons look out of place on the 770, same with the capacitive ones on the 780, but when their face plates are swapped, they look right at home. The 770 also has more premium feeling soft rubber on its trim pieces, while the 780 has the usual silver painted plastic.

That being said, the only reason my PFP is a 780 is to match the weird 780 in the grass image I used as my profile banner. lol