If done right, it would be possible to create deck building challenges which would contribute the growth of new players. Let’s take the color mastery alone as border setting device, and say: bronze tier event only, and make it a three rounds, three nodes challenge, revolving around a simple building concept as
1a. “Win with a deck that has 5 or less creatures” (red or green)
1b. “Win with a deck that has 3 or more spells” (blue or black)
1c. “Win with a deck that has 1 or more colorless cards (multicolor)”
If you win all three, you get a common or 10 crystals or a basic booster in the color of your choice
If you won two, you get 5 crystals or a basic booster
In the second and third round the difficulty grows a bit and the prizes grow accordingly:
2a. 4 or less creatures,
3a. 3 or less creatures
2b. 4 or more spells,
3b. 5 or more spells
2c. 2 or more colorless,
3c. 3 or more colorless
Uncommon or 1000 mana runes or 15 crystals (and so on…) - give the players the right to choose their prize.
Run this every two or three days.
And then every two weeks make a NEW challenge, in order to make the deck building less dull.
Complete newbies could be able to manage the first round and getting a basic booster extra is already great for the beginning. In the third round there could be even a rare waiting if you manage to win all your games. This is highly optimistic though. Those somewhat more advanced would have the challenges of rounds 2 and 3 and so on.
Generally speaking, I believe the new players wouldn’t have to suffer under deck building events if they are made right. My example is very simplistic, there are definitely more fun tasks to build around that a newbie could manage. Important thing are the prizes. If we want people to stick around, then they should have the sense of growth thru winning and earning prizes and getting better cards.