From my understanding the color simply designated the engine,
red 323 with a 3hp
yellow 424 with a 4hp
blue 525 with a 5hp
The 4 or 5 could have been a single, dual, or tc so it could have been yellow or blue. I'm sure there is one out there to prove me wrong but I've only ever seen a 323 with a single speed.
These things are starting to look like the copo chevys of the 60s. There is a list of what is supposed to be but it seems if you knew how to check the right boxes you could get any combo of the parts you wanted. The way parts interchange its too easy for somebody to build what they want and claim it was like that factory. That alone is somewhat a pet peeve of mine. I don't mind somebody restomodding but don't try to pass it off as some rare one of a kind original.
I know that the color coding for the engine was a thing on the MW bikes:
red for 3hp
yellow for 4hp
blue for 5hp non lighted, single speed
black for 5hp lighted, dual speed
green for the trailbike, TAV
but I'm not sure that applies to the Gilson branded bikes.
The ad in '71 shows a yellow trailbike. Note, the second bike listed is a 6" wheel TAV model, but no color is listed
The same ad ( i think it's from the same ad) shows all models (all engine sizes) in orange
Another ad that I found a while back shows the model that's currently listed here,
and it's blue, but i can't make out if it has the TAV or dual speed, or single speed clutch...
I agree there are some discrepancies among what we know (think?) to be true. Maybe some day we'll put all the pieces together. These bikes are growing in popularity, as they well should. The more popular they get, the more answers we'll have