I think you should definitely be able to do what you say using both methods...in fact i just tried the same on a report and it worked
I added a column to the SQL, then added an assignment to set the color to red using the style wizard and set condition {somecolumn} > 7 and then used the Columns button to show the columns and selected comecolumn and chose to hide it. All seemed good.
Are you saying that the conditional formatting works but when you try to hide the test value it stops working as per your description? It sounds like your test column has got corrupt. Can you double check your SQL still has the column in it and it matches the column for the assignment .. if you unhide the column does it still do the formatting?
If you want to upload the xml file here or send it to me at info @ reportico.org that would be very useful
Do you know which version of reportico you are using and whether its for LAravel/joomla or standalone?