Hey all, I am still alive, just not really posting on this site, same as all of you I expect... I did find some interesting photos of some Korean merch from 2000 (in a collection of Blizzard-related merch in The Strong National Museum of Play in Rochester, NY) that has some color and colorized versions of the original SC1 manual renders. Some of these are definitely just colorized, like, the original color render of the Scout in the manual was brown and pink, and in this version they've colored it all yellow. The dragoon definitely and also probably the high templar are also just colorized yellow with some accents. But for the terran and zerg units here, I think these might really be the real colors of those models:

It's also possible that they're all just colorized from the black and white versions, and some of them are a better job than others. Like in this one, we know those goliath colors are not accurate, and a lot of the vulture still looks pretty grayscale... But it's possible that the difference is that they were recoloring the cinematic model ones to look like they had team color and colorschemes like the in game units. idk.

But I think the Zergling is authentic because, as you can see from their attempt at colorizing the cinematic guardian model, the colorized ones don't look that good and have much more blocks of solid colors.