You need to make sure Show Previous Tax Year Color is check in 2020.  Then restart program.

ALSO, 

1. 2021 must be present.

2. File can't be locked (or you'll see gray background)

3. You need to be in Forms Mode rather than Quick Entry Mode. This is controlled by "Entry Mode" button towards the top of the program. There is no background color change in Quick Entry Mode.

Let us know if you still have problems, along with some more details, such as Pro vs. Basic, Network vs. StandAlone, and version.   See Help-About->Version. You are looking for a number like wPro.2020.21.00.14 (not necessarily this number! example only)

And anything else you think might be relevant or provide a clue.  We'd like to get this working for you, as it is a useful feature.

Also note that to prevent changes to 2020 files (or 2021 for that matter), you can lock the client file. Highlight a client in Homebase and go to Tools->Lock/Unlock Client Files