IF "Green" is picked from down down list clear other cell

Excel:

In D23 I have a drop down list with the options of Red, Amber, Green, Complete, Cancelled, On Hold. In E23 I enter a date of when the project will go Green if it is in any of the other states.

I would like E23 to be empty if D23 is Green. Otherwise E23 should be able to have a date typed in i.e 10/02/2016... any ideas how to do this?