The RX's button and logo are actually a pale purple.
Otherwise, they're the same system.
What the R & RX are missing from the original Duo is door lock, earphone jack & volume knob, expensive black/grey plastic and the AV port is now centered at the back instead of on the side(essentially widening the overall system by 3 - 4 inches) where it's easier to get stepped on/wacked.
When I got my first Genesis back in the day, I built a small wooden stand for it that housed the cart caees below. After getting a Duo years later, I had to drill a hole in the side of it to connect the AV cable and it was then sitting where the Genesis cases used to go. If it was setup like the R/RX, I could've just placed it on top like a Genesis.
The ironic thing is, one day I actually wound up stepping on the AC adapter port instead and ripping a circuit board loose. Of course, theis was because I had the system on the floor instead of in my stand.
One last thing, I've heard a lot of people say that the earphone jack was useless, but I used it all the time. Back in the day, I had old fashioned TV's in my room, which I had to use a vcr to run the Duo's AV cable through to connect to.
Since I didn't have a fancy stereo and couldn't crank up the volume at my Dad's place anyways, I bought an expensive headphone set and always used it when playing Duo games on my own.
I actually miss that setup now, since I got to here the sound quality so much better than I do now through my modern TV's or stereo system.