I noticed that the only fill color that you can change is the fill of the stars in the rating. You cannot change the fill color of the empty stars, or did I miss something?
If here you means the rating control, then yes, there is no Fill property available to change the empty stars in Rating control.
Fill property: Background color
RatingFill property : Selected Star color
Rating control in PowerApps
You may consider add an idea for this, to suggest add a Fill property for empty star :
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Features releasing from October 2019 through March 2020