Was there in January and yes, I think you are correct.

As I recall there are four colours

1) For Doha (Blue I think)
2) For transfer (Yelllow I think)
3) For fast connection (yellow and orange)
4) For premium terminal (burgundy)

It's one bus to all locations.

The confusion arises if you are a premium passenger with a fast transfer, as we were. We were issued with the yellow and orange boarding card and there was some confusion on the bus as to whether to get off at the fast transfer stop, or the premium terminal. Even the staff didn't know. (We opted for the latter btw)