Are you Dave?

You could use Format|Conditional formatting.

Select your range (I used A1:X99)
then with A1 the active cell
Format|conditional formatting
formula is: =CELL("protect",A1)=0

Format it nicely.

If you're Dave, you have a response at your first version of this post.

Questionable wrote:

If you have protected a worksheet, how can you highlight an unprotected cell
on that worksheet?

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Dave Peterson