Hmmm, I’m not sure how much help changing the colour will be. Perhaps when you first add / edit the date it might stand out to you. Otherwise, you still have to be on the ‘right’ page or record as you were today.
Still, it’s super simple to add this, so I’ have done it, and it is in time for the next release.
I did briefly wonder if it could be included in either Gedcom check, or Sanity checker. But in both cases the resource load needed to check EVERY date in your tree would be far too great. Those tools only work if the checks can be targetted in some way.
Gedcom check is limited to links and high-level Gedcom rules.
Sanity checker is limited to specific events or pairs of events.
Invalid dates can occur in any record and any event within any record, so potentially hundreds of thousands of checks to do!