The next step is to add a layer mask to the existing “Rainbow” layer. Make sure you have the “Rainbow” layer selected. You will find the “Add a Layer Mask” icon at the bottom of the Layers palette. By clicking it this option, you will add a layer mask thumbnail to the right of the “Rainbow” layer thumbnail in the Layers palette. You will notice that the layer mask thumbnail is filled with white. The layer mask is selected if the thumbnail has a white highlight box around it.

You must not choose the Black-to-White Gradient.By pressing the D key you will set your Foreground color to white and the Background color to black. After having done so, press the X key to swap them.

Grab the Gradient tool form the Tools palette and select the Gradient Picker. On the top left corner of the Gradient Editor you will see the gradient swatch in the top left corner, and select the Foreground to Background gradient. Now go back to the Options Bar and select the Linear Gradient option.

You must now drag a Black-to-White gradient from the base of the rainbow in your picture to the top of the image. You need to do this in order to give your rainbow a more realistic look, as real rainbows tend to get brighter at their top. The way to obtain this with Photoshop is by dragging a black-to-white gradient from the base of the rainbow, the point where it reaches the ground in your picture, to the top of the image.

At this point you will also want to cut the rainbow’s base, so that none piece of it which is below it base will appear in the final image. Select the Gradient tool. Now click the mouse somewhere around the point where your rainbow appears to be touching the ground. Hold the Shift key and drag straight up to the top of the picture, After you release the mouse, Photoshop will draw the black-to-white gradient on the layer mask and by doing so, your rainbow will appear to be starting near the bottom of the picture and gain in brightness as it climbs up in the sky.