Once you skip the color challenge, you will have access to battle other players. This option will allow you to work around the bug.
Disclaimer: I have not tried this on PC. It does work on iPad and iPhone.
Step one: Click the play button in the bottom right.
Step two: Under “Find Match” select play.
Step three: Click play at the bottom right.
Step four: Immediately click cancel before it finds an opponent.
Step five: Click play at the bottom right again, but this time select color challenge.
You should now be able to get a match and complete the 5th step of the challenge. You will need to repeat this process for each color. Also, after step 4, I was still able to complete a challenge by clicking the color challenge bubble and starting the match through that route.
courtjester#32998
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Seemingly working workaround, copying from Reddit:
I, like others, have been unable to complete to color challenge. Wizards has recognized the issue and provided instruction on how to skip the challenge here: https://twitter.com/wizards_help/status/1398405042672222211?s=21
Once you skip the color challenge, you will have access to battle other players. This option will allow you to work around the bug.
Disclaimer: I have not tried this on PC. It does work on iPad and iPhone.
Step one: Click the play button in the bottom right.
Step two: Under “Find Match” select play.
Step three: Click play at the bottom right.
Step four: Immediately click cancel before it finds an opponent.
Step five: Click play at the bottom right again, but this time select color challenge.
You should now be able to get a match and complete the 5th step of the challenge. You will need to repeat this process for each color. Also, after step 4, I was still able to complete a challenge by clicking the color challenge bubble and starting the match through that route.