I want to be able to keep track of a number of incoming documents in excel
and ensure they are actioned within a given timeframe. I want to set up a
simple spreedsheet with columns named: action, date in, date due out.

I want to use colours to identify the urgent requests - ie red for a
document that is within 3 days of its due date, orange for something that is
5 days within due date and green for actions that have greater than 10 days
until its due date.

I assume i need an action for this, but am not entirely sure how..?