IsEmpty ( Table::SpecificFieldHere )
Then specify a fill color for the button.
I think I will need to do this in reverse, "If not empty" change the colour. It seems to be picking up an empty field at the bottom of the portal.
The expression needs to explicitly reference the field that you want to control the color of the button. You could reverse the logic with Not Isempty ( instead of IsEmpty, but if the expression that I recommended didn't work reversing the expression isn't likely to work either.
The expression references one specific fields so the fact that another field is or is not empty should have no effect on how the conditional format works.