If formula based on cell colour

should have said that in the first place :D

That's much simpler, requires no VBA to work as ISBLANK() is already a worksheet function.

Basically, change the conditional formatting to:

=ISBLANK($G3)

Because a typed value triggers a worksheet change, this will always change when needed. The fill color doesn't qualify as a worksheet change event apparently...

See Attached sheet :D