I have a spreadsheet where I document correspondence received, and I have a column "Days open" with a basic formula to work out how many days the case has been open for. We split the senders into two categories: important and unimportant (I have a hidden column titled "important" with yes or no in the cells). The unimportant ones are allowed up to 15 days to be closed, and the important ones up to 5.

How can I format the column of "Days open" to change colour after 5 days for the important letters and after 15 for the unimportant ones?