I found another way to trigger the color change, you can try this if you like. It worked for me.
Kris
OK, so strictly if due date is in the next calendar month. Could even be up to 61 days away... Due 5/31, it should be triggered beginning 4/1 - 61 days.
And it should stay colored beyond due date until a new due date is entered?
Say Due date is in B1
Select B1
Click Format - Conditional Formatting
Select "Value Is"
Select "Less Than Or Equal To"
Enter this formula
=DATE(YEAR(TODAY()),MONTH(TODAY())+2,0)
Click Format
Click Patterns
Select the color you like
Click OK
Click OK