There are multiple ways to do this:

1. Shift + G will activate the Paint Bucket tool.
2. Shift + Backspace will fill with a color of your choice
3. Alt + Backspace will fill with the foreground color you have selected.
4. Ctrl + Backspace will fill with the background color you have selected.
5. You could also double click the layer and add a color overlay in the effects panel.

Hope this helps