Now it is time to group the two original rainbow layers if you want the rainbow to appear somewhat behind the elements in your picture (if you, for example have some trees in the image and you want the rainbow to appear behind them). Select one of the two “Rainbow” layers in the Layers palette and then Shift+click on the other one to select them both. After you have both the rainbow layers selected, press Ctrl+G to place them into a group. When you have more layers in a group, you will see a folder appearing in the Layers palette where the layers have previously been.

You can add a layer mask to a group. You can do the same way you add one to a layer. Having the group selected, click on the “Add A Layer Mask” icon (which is located on the bottom of the Layers palette) and a layer mask thumbnail will appear to the right of the group's folder icon.

The next step is to paint with black over any part of the rainbow you want hidden. Use the “Brush” tool from the Tools palette (or press B if you prefer working with keyboard shortcuts). Select a soft-edged brush and black still as your foreground color, paint over anything you want the rainbow to be hidden behind. ( if you have some trees in your picture, paint over the trees just to make sure that no part of the rainbow is visible in front of them). Because you paint with black on the layer group's mask, both of the original rainbow layers inside the group will be hidden from view in any areas you paint over. In the end, your picture should look like this:

After you have painted the elements you want to appear in front of your rainbow in the picture, you are finished editing the image.You now have one or two rainbows with a very natural look in the picture. Save the file with the desired name and extension (File > Save as)