You could make a new layer (in case you want to keep what was there originally) and go to Edit > Fill. If you don't have the color you want selected already (as either the Foreground Color or the Background Color), you can change Contents in the Fill dialog that opens up to Color and use the Color Picker that opens up to select the color you want. Click OK for the Color selection and click OK again for the Fill.
Thanks, Myra. I appreciate your wisdom of modifying a duplicate layer, and possibly flattening layers once finished. What command or menu sequence makes the animated borders of selected areas disappear permamently? 'Okays' via Edit>Fill Selection>Color don't eliminate/stop those animated borders for me. Thanks. ChrisD