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Preserving Transparency in Photoshop
If you have a layer that you want to fill with the foreground colour, the keyboard shortcut is Alt + Backspace. To fill with the background colour, use Cmd (Ctrl) + backspace.
If you want to only fill the pixels on the layer, not the transparent areas, you have a couple of choices; either you can add the Shift key to the previous key combinations or you can lock the transparent areas. Press the forward slash (/) key to lock the transparent pixels and you can target only the opaque pixels on your layer with brushes, gradients, and fills.
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