It all depends on how you want to handle NULLS on either side, but the test for a NULL is "ISNULL([Dim])"
So in the attached I've done
If Dimension1 ="Red" and (Dimension2<>"Yellow" OR ISNULL([Dimension2]))
Hope that helps, and if not 100% give you the logic to amend it.
Handling Null Values in Dim.twbx 25.8 KB
Thank you very much, ISNULL worked for me