I just noticed something in my timesheet, an Excel workbook I use to track my hours for clients. On the status line there's a notice saying "Accessibility investigate"; I now realize it's been there at least 24 hours but I have no idea how much longer ago it might have first appeared. When I click on it, a list appears consisting of two sections. One is labeled "Errors" and has just one line under it, "Exclusive use of color". The other is "Warnings", and the sole item under it is "Merged Cells".

The second item points to place where I merged two cells in an unusual way; I suppose Excel is pointing it out to me in case it's an accident. (It isn't.) But I can't figure out what "Exclusive use of color" might mean. Anyone know what I should look for? There are 26 items in that list, many of them for multiple cells. I haven't looked at all 26, but the ones I've examined so far don't seem to have any color involved at all.

The accessibility checks for things that might be a problem for users with issues such as colour blindness. Things like using a red font for negative numbers can be an issue and get flagged.

I think Exclusive use of colour is where the colour is the only thing in cell(s) which signifies something. Eg. You might be using a red colour to signify a negative number:
A positive number might look like:
33
and a negative nuimber:
33
where there is no negative sign, so the only thing signifying negative is the colour.