See it you can set up conditional formatting on the tab panel to change it to grey if the fields are empty.
If that doesn't work, you might replace the tab control with a slide control with a button bar replacing the tab labels. The buttons in the button bar can use go to object to bring a specific panel of the slide control into view and you can then set up conditional formatting on each button of the button bar.
Tab labels as well as placeholder texts or button bar labels can be formatted individually using text formatting functions.
For example :
case ( isempty ( yourfield ) ; textColor ( "the tab" ; rgb ( 255;0;0)) ; "the tab" )
Just checked. Conditional formatting works as I recalled that it does for tab controls.
Click to select the tab control
Click again to select a specific panel
Right click and chose conditional formatting or chose "conditional..." from the format menu.
You can then set up a conditional format expression to change the text color of the tab label.