I will try to explain using your screenshot.
Value Fill is the white line And
Handle fill is the Round white at the end of the white line.
So if you have a Value fill property of Age.HandleFill, it means the color of the value Fill should be the same as the handle fill. So if you want to change the Value fill color instead of Age.HandFill, replace it with for example Blue.
You can make reference to the documentation;
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