Matching image to background using Curves adjustment

Turn off the clipping mask and select the background then turn back on the clipping mask when you are done.

If you are using the eyedroppers within the Curves panel, the black point eyedropper sets the pixel you click on to black or near black. The white point eyedropper sets the pixel you click on to white or near white. The graypoint eyedropper sets the pixel you click on to a neutral color (a shade of gray).

If you want to sample a color, click on the layer that has the color you want, to make it the active layer. Then use the eyedropper tool in the Tools panel to select the color you want. That color will then become your new Foreground color and you can use it to fill an area (Edit > Fill).