Once you install 2021, then the 2020 background color changes, but not any other years.   There is no option to choose a random color for the screen.

From What's New:

https://proconnect.intuit.com/community/proseries-tax-news-updates/discussion/proseries-tax-2021-ins...

"Previous Tax Year Color:  Easily differentiate between the current year and prior year programs while working. Enable or Disable the feature in ProSeries Options"

Before 2021 changed 2020 forms, you could do something else for prior years:

Remove the color from the input fields.

Tools Options uncheck box to remove field color.

At least it made it harder to see the input fields. 

Guess I've been way ahead of the curve on this.  I normally have current and prior years returns open at the same time, and often would be inputting on the wrong one.  Then i found this easy fix:

I guess @Accountant-Man was more header of the curve than me....see his post above

Tools>>>>Options>>>>General>>>>Like third line down... to the right "Show Field Color".  uncheck that box....  You can certainly determine which year you are in..