Well the prometheans aren't the covenant, so I dont think it has to do with that if their simply machines being used by forerunners.
That's not for sure, we don't know if the orange means corruption, all we know is that there are two variations of the promethean classes we will be battling. Yes while the clues do lead to the Didact being "unbound" i think it's more to it then just "they're corrupted"
The blue enemies were still prometheans, they were just colored differently, so it wouldn't make sense if the builders were in control of them. If anything, I would expect it to be a promethean. The Didact seems likely, however I'm interested in his friend the Confirmer. In Cryptum, he was the guarDian of the San'Shyuum home world, and over time, he grew to like the species. However, he went on to guard the world by himself for 1,000 years, and deteriorated, both physically and mentally. So imagine this, you like this species, and their nice to you. But the flood causes the forerunners to activate the halo arrays, almost causing the race you like to be extinct. You are angry because what drove the forerunners to do this is to preserve the mantle, or save humanity. You then hate humanity, and wait for millenia for your chance to strike back. This would cooperate with the line "I have long dreamt of this day reclaimer." and it could mean the orange prometheans belong to him, and since the Didact has been all but confirmed to be in the game, he could be leading the blue ones, and end up fighting both the Confirmer and us for unknown reasons. Guess we'll just have to wait 'till november 6