Apply Conditional Formatting To Highlight ONLY New Content On Column

I have a spreadsheet and I want to color particular cells in a column with a new color - i.e. any new changes need to be highlighted. I know there's a way to do tracking changes in excel, but it just sticks a little flag almost invisibly in the corner of the cell. I want to be able to bring the spreadsheet back to our administrator and say hey the stuff in red is new.

On a related note - I am working on this massive spreadsheet that is a .csv but I am saving it as an exel spreadsheet - is that ok? I am assuming that if I save it as a csv, it will return to the original formatting just without the colors, filters, etc changes I made - which is fine because I think somehow the .csv file will be uploaded to the system and no further changes need to be made.

I found out the hard way when you have a .csv file and make changes and then save it, you lose all the fun row/column size adjustments, color, etc - but I figure in the meantime I'll work on it as a excel spreadsheet and then return it to it's natural .csv file status.