But Powered Convoy isn't black/dark gray.
No, but I never read Powerlinx Prime's colours as black or dark grey. I still have my toy from back then. He's on a shelf in the other room. He's navy blue, and so I read the cartoon's colouration as navy blue for the body and royal blue for the hands and head.
Because of this the colour scheme was always "a blue Optimus" in my head and "blue Optimus" just defaults to "Powered Convoy" to me. Being a darker blue than traditionally used for that scheme is what made it seem like a stealth homage to Primal as well, but my mind went to the Diaclone colour scheme first. Probably because I'd just learnt about Powered Convoy around that time and thought it was very, very cool.
The Takara release of Powerlinx Prime is also called "Magna Convoy," which is one of two modern names HasTak likes to use for the Powered Convoy deco these days (alongside Delta Magnus). I know that the "Magna" in that Takara release actually refers to the packed in Overload, who's called "Ultra Magnus" in Japan, but still. It just further reinforces my earlier mental association.