If you have a single layer in Photoshop, you can add an adjustment to it by using the Adjustment Layer dialog. This dialog allows you to change the brightness, contrast, and saturation of the layer. To add an adjustment to a layer in Photoshop, follow these steps:

  1. Open the Adjustment Layer dialog.
  2. In the left pane, select a color or tone.
  3. In the right pane, select a value for the adjustment.
  4. Click OK to add the adjustment to the layer.