Step 11: Duplicate The Rainbow Layer To Increase Its Brightness

To increase the brightness of the rainbow, simply press Ctrl+J (Win) / Command+J (Mac) to duplicate the layer. If, after duplicating the layer, you find the rainbow is now too bright, dial it back a few notches by lowering the opacity of the duplicated layer. You'll find the Opacity option directly across from the blend mode option at the top of the Layers panel. I'm going to lower mine down to around 70%:

Here's my image after duplicating my rainbow layer and then lowering the opacity:

Step 12: Group The Two Rainbow Layers

Let's do a little housekeeping in the Layers panel by grouping the two rainbow layers together. Grouping related layers makes it easier to keep the Layers panel organized. With the top layer selected, hold down your Shift key and click on the original Rainbow layer directly below it to select both layers at once. You'll see them both highlighted in blue, letting you know they're both selected:

Then go up to the Layer menu at the top of the screen and choose Group Layers:

Photoshop will group the two layers together in the Layers panel. Photoshop names the new group "Group 1". By default, the layer group will be closed, preventing us from seeing the two layers inside of it, which is fine for now:

Step 13: Add A New Blank Layer

We have our initial rainbow. Now let's add a second one to create a double rainbow effect! First, we'll need a new layer, so click once again on the New Layer icon at the bottom of the Layers panel:

The new layer will appear above the layer group in the Layers panel. Double-click on the new layer's name and change it to "Second rainbow", then press Enter (Win) / Return (Mac) to accept the name change:

Step 14: Re-Select The Rainbow Gradient

Click once again on the Gradient Picker in the Options Bar at the top of the screen:

When the Gradient Editor opens, click again on the Russell's Rainbow thumbnail to select the rainbow gradient, but don't close out of the Gradient Editor yet. We need to make some changes to it:

Step 15: Flip The Order Of The Colors In The Gradient

Whenever we see a double rainbow in nature, the colors in the second rainbow are always in reverse order, which means we need to reverse the order of the colors in our gradient. With the Gradient Editor still open, click on the blue color stop directly below the gradient preview bar and drag it towards the left. Then do the same thing with the green and yellow color stops, dragging them towards the left. It doesn't matter how far you drag them as long as there's some space between them. This is just to make sure they don't overlap each other as we reposition them. Leave the red color stop in place:

Click on the red color stop to select it, then enter a value of 82% into the Location option. This will place the red color stop in the same location that the blue color stop was in originally:

Next, click on the yellow color stop to select it, then set its Location value to 85%. This moves yellow into the spot originally held by green:

Click on the green color stop to select it, then set its Location value to 88%, which moves green into yellow's original location:

Finally, click on the blue color stop and change its Location value to 92%, placing it in red's original spot:

Directly above the gradient preview bar are more stops, filled with white, black or some shade of gray. These stops control the transparency levels (opacity) of the colors throughout the gradient. Click on the light gray opacity stop (second from the left) to select it, then change its Opacity value from its original 20% to 0%. Click OK when you're done to exit out of the Gradient Editor: