Q:

Conditional formatting conundrum based off two dates in Google sheets

I am using Google Sheets.
I have a lot of columns. The only ones that count are: a column with dd/mm/yyyy named "start date" and a column with dd/mm/yyyy named "finish date".
I want to:

highlight red if there is a start date but no finish date.
highlight orange if there is a finish date but no start date.

I haven't been able to get a formula (in conditional formatting) to work.

A:

for orange applied to range A1:A:
=(A1="")*(ISDATE(B1))

for red applied to range B1:B:
=(B1="")*(ISDATE(A1))

