No offense! but isn't the correct answer to the question "How do I change the Marketing Channel Color (...)? should be below
Go to report suite > Marketing Channel > Marketing Channel Manager > Channel color (last column in your screenshot).
Click on each Color Box to select the desired color option (27 options) > select a different color for each marketing channel.
Don't forget to hit save to apply the selected colors.
I know due to bug it's not working currently but it should not change the answer of the question? Lmk
Ye, this seems to be a bug. But I just found a turn around. Copied all the Marketing Channel setting from dev report suite (setting this as template) to prod suite. This resolved my problem.
Anyways, thanks for helping me.