Go to your column header (difference to today) and click on the header.
select “cell Formatting” option and give your range like min=0 and max 1 and background color green; Min=2 and max=3 then and background color ; max = 5 then and background color Red.
Ganesh is right: the only solution is to create a calculated measure that calculates the days difference, then add this measure to the report and colorcode cells in that column (instead of the exact date).
Currently, cell formatting could not be applied based on logical conditions or functions (days difference, color accordingly to the value in another column etc). Though, we have plans to improve this functionality (no ETA yet).