Fill a Layer with a Solid Color?

If you want to fill a layer with a solid color, click on the Foreground color swatch at the bottom of the Toolbox to open the Color Picker, choose the color you want to fill the layer with, and click OK. Then, press Option-Delete (PC: Alt-Backspace) to fill your current layer with that color. If you want the color to be see-through, then go to the top of the Layers panel and lower the opacity of the layer using the Opacity slider.