Add A Realistic Rainbow To A Photo In Photoshop

Step 6: Apply The Gaussian Blur Filter To The Rainbow

We're going to make the rainbow look more realistic now by blurring it quite a bit using Photoshop's Gaussian Blur filter. To do that, go up to the Filter menu at the top of the screen, select Blur, and then select Gaussian Blur. This brings up the Gaussian Blur dialog box:

Drag the slider bar at the bottom of the Gaussian Blur dialog box to the right to increase the amount of blur being applied to the rainbow, and keep an eye on your image as you drag so you can see a preview of what's happening. Continue dragging to the right until your rainbow looks more realistic. Depending on the size of your image (the pixel dimensions), you may want to set the blur Radius as high as 50-60 pixels. I'm using a low resolution image for this tutorial, so for me, a radius value of around 25 pixels works well. Click OK when you're done to exit out of the Gaussian Blur dialog box.

Here's my image after applying the Gaussian Blur filter to my rainbow:

Step 7: Add A Layer Mask To The Rainbow Layer

With the "Rainbow" layer still selected, click on the Add A Layer Mask icon at the bottom of the Layers palette:

This adds a layer mask thumbnail, filled with white, to the right of the Rainbow layer contents thumbnail in the Layers palette:

The layer mask itself is now currently selected, which you can tell by the white highlight box around its thumbnail.

Step 8: Choose The Black-To-White Gradient

Press D on your keyboard, which will set your Foreground color to white and your Background color to black, then press X to swap them. Grab the Gradient tool once again from the Tools palette if it's not still selected and then click again on the Gradient Picker in the Options Bar:

This will bring the Gradient Editor back up. Click on the gradient swatch in the top left corner, the Foreground to Background gradient, to select it:

Step 9: Choose "Linear Gradient" From The Options Bar

Go back up to the Options Bar and this time, click on the Linear Gradient option:

Step 10: Drag A Black-To-White Gradient From The Base Of The Rainbow To The Top Of The Image

A real rainbow would normally appear to get brighter as it reaches higher into the sky, so we're going to give our Photoshopped rainbow that same effect by dragging a black-to-white gradient from the base of the rainbow (the point where it appears to touch the ground) to the top of the image. By doing this, we'll also be cutting off the rainbow at the baseline so none of it that's below the base remains in the image. To do that, with my Gradient tool selected, I'm going to click my mouse somewhere along the bottom of the trees on the left, which is where I want my rainbow to "touch the ground", and then I'll hold down my Shift key and drag straight up to the top of the image. Holding down Shift forces me to drag straight up, preventing me from accidentally dragging a little to the left or right:

When I release my mouse button, Photoshop draws my black-to-white gradient on the layer mask, and my rainbow now appears to start near the bottom of the trees on the left and gets brighter as it reaches higher into the sky: