On the label component add props.style.background
(edited, thanks, @pascal.fragnoud) and create an expression binding to the variable to be displayed. In my case it's the slider.
"linear-gradient(0deg, #ff0000 0%, " // start red
+ "#ff0000 " // continue to cut-off
+ {../Slider.props.value} + "%, "
+ "#ff000000 " // then transparent for remainder
+ {../Slider.props.value} + "%)"