How to make LGBT Rainbow Effect for a Black & White Image

Trevor provides some great options for replacing the white portion of an image. I would also like to add an option of using the Select Color Range tool. 

It can be found by going to Select>Color Range

Here is adobe's section on it:

Select color ranges in Photoshop (adobe.com)

And a great video of it in action to get a very precise cutout/mask:

Precisely Cut Out Hair Using Color Range in Photoshop! - YouTube

Beware that some color modes including Grayscale mode won't allow you to have color gradient. Safe option is to check that Image > Mode is set to RGB before adding gradient.

Another option, and perhaps a better one, is to place the rainbow layer above the B&W image and use Blend If

Double click the B&W layer to open the Layer Styles panel, and use the This Layer slider moving the right slider to the left.  If you hold down the Alt (Opt) key you can separate the right slider, as shown in my cyan highlight, to get more control over what tonal values will become transparent to reveal the rainbow in the layer beneath it.  You can see that this method has eliminated the slight fringe that shows in the layer mask version.

You can't use colorize, or the color blend mode over white.  It just won't show.  So you'll need to select the wite area and use a mask. 

Place the rainbow in a new layer above the B&W image.

Turn the rainbow layer off and Select the white area with the magic wand tool.

Turn the raibow layer back, and give it a layer mask.

I am sure you'll know that there is already a rainbow gradient available.  If you open the gradients panel you can use the search bar to find it as gradients are kind of hidden away nowadays.  My green highlight is the add layer mask icon.

Note I have used the radial gradient (my red highlight

Hi Clay!

If you could share a visual example with us, we could give you some great tips on how to approach you request.