Anthony Rudd said:

.When I click a cell, I see the associated text in the Edit window.

Then we are definitely NOT talking about hidden columns, since if a column would be really hidden (as per the Excel terminology), you wouldn't be able to click the cell in the first place!.

So you are most probably talking about something like "invisible text", i.e. a white font color on white background, or something similar... Which is a completely different thing than hidden text/columns.

Thanks, sometimes the answer is "obvious". But this does not explain where this new colouring came from.

That can be found out by analyzing the original Excel content and the content in Studio, using some experience and knowledge and looking for suspicious things...

Perhaps the coloring is there already, but your Studio Editor setting shows only the text without formatting, i.e. you thought it's not there?
Hard to say without knowing every possible detail about the files, your settings, habits and processes.