As a new player I find the color challenges may a bit polarizing for some. Personally, I enjoy the idea that they are self contained puzzles, simulating common scenarios one may find in a match, specific to their deck. It doubles as a crash course in the structure of the game as a whole, as well as introducing the specific mechanics the deck colors may use.

On the other end of the spectrum new players may be put off by the trial by fire type play these color challenges have. Trial and error reveals just at what turn, what cards need to be played where, and while i found this process somewhat enjoyable, others in my new group find it quite frustrating. They also pointed out that the hints are also somewhat unhelpful, or rather, they are a bit cryptic, in that they give you a slice of the answer, but not the whole outline of the solution... hmm again I think this is a fun way to some, but may also be frustrating to those that just want to complete the color challenges and start collecting and playing with the new decks.
With progression tied to the completion of the color challenges, we really wanna make sure we get people through this specific process, seamlessly, without frustration, but also with the knowledge of the basic game structure.

A potential solution I recall, was something found in the mission trials of Guilty Gear Xrd. In it, players are tasked with things like defending certain sequences of actions from a specific character, and couter-attacking. In this, players learn character specific match-ups, what to look for, as well as what their response should be in a scenerio, specific to their character, to condition a response.

So in a similar way to color challenges, this is a trial and error type challenge, in that the player repeatedly fails, until they can arrive at a series of actions that produces success.
Specifically of note are the hints GGXrd gives the player when they are trying to complete these trials. There is a general hint the player receives upon failure (MTGA does the same).
However, there is a secondary, much more intricate, explicit hint that the player can view, upon repeat failure. To do this, the player is presented with the hint prompt, and can then choose to view it at a button press.
For those hot heads, that just wanna clear a challenge, or those simply in the dark, this is a nice way to give them that extra push to persevere and finish. And for those who like to continue grinding with the trial and error method, they can ignore the hint and keep trying.

I suppose it is a small thing, but considering this is the first gate a player must pass through, it can be a bit discouraging to admit defeat, then have to go watch some video on youtube about how to complete the trials. That sense of accomplishment in finding the solution, is part of that response we want to condition the players to have after all.. the ability to recognize small victories in playing cards correctly.

All that being said, I like the Color Challenges, and truthfully ide like to see more similar content with more complex decks.
I consider it to be like a puzzle, a type of mini game, and also it is single player content which, in competitive games is important. It is taxing to a player to jump in and get beat down, so having that option to warm up, or grind out a small puzzle to cool down, is a nice change of pace for casual players.
It is a great way to learn to play a deck, to understand cards, and understand more intricate rules.. Though i do realize it takes much time and consideration to craft these scenarios. Perhaps it may be good to simulate common scenarios one may find when playing .

As a secondary thought, and again, I preface that I am a new player;

A bit of an explanation of things such as the general play of a deck, or the main systems at play in a deck or common to a color, would be quite helpful.

For example:
Mutations / Scrying / Sacrificing / ect. archetypes to a deck.

While a player can understand these slowly as they play a deck and learn how it wants to be played, general outlines within MTGA itself may be helpful to new players.
Again, i think there are plenty of resources out there, for those who want to dig, but in this idea of game design i find it in your best interest to keep the player within the bounds of the game. The more information they can find within the game, the more they can interface with the product itself. Essentially, it is that fear a casual player has, browsing forums that lead to accel sheets and complex charts, when all they really need is a broad answer to a simple question.