Continuity mistake: Optimus Prime, whose primary color is red, when being tied to a rock alongside his troops during the episode's climax, turns blue for a second.
Continuity mistake: After the Decepticons dig up Skyfire they look at his memory banks to show what happened to him. In the flashback it shows Starscream transforming into his Earth jet mode which he shouldn't have had because it takes place before they all crashed on Earth.
Continuity mistake: Skyfire's Decepticon symbol changes color throughout the episode. Sometimes it is purple and sometimes it is red.
Answer: This was actually one of many errors in Transformers, the Constructicons MUST have been built on Cybertron, the only transformers genuinely built on earth were the dinobots, reason being Vector Sigma has to give personalities to each and every robot, else they end up like the dinobots, very simple and dumb. For reference watch the episode "The Key to Vector Sigma" which introduces the Stunticons and Arielbots - both Megatron and Optimus Prime have to go to Vector Sigma to get decent personality traits installed, prior to this the new robots are controlled by remote controllers and have no AI.