Add a small colour wheel to the main playing area (maybe only for the first few challenges per game?). It's on the pause screen already, but I know when things got hectic I'd have liked it to be there in front of me.
The single colour challenges were pretty punishing, in that there really weren't enough bars coming of that colour to push through to the next challenge. Perhaps give a small amount of time for non-challenge related matches?
Both points are great feedback on elements that I think we've struggled with during development by being so close to it.
I think we've overestimated peoples knowledge of the color wheel. The data is certainly showing it to. Scores from our beta rounds were much better than the first couple days. My assumption there is the beta users got used to the color wheel over time. Your idea of a hint during the first view rounds is a great one.
Also, for the challenges, were you only trying to complete the challenge? This is another point that I think users are struggling with. Completing the challenge is a bonus, but it's certainly not the only goal. If the challenge says "Clear 10 sets of 5", you can still clear out sets or runs of 3 until you are blue in the face. Challenges aren't required.