How to use keyer as a mask ?

I have a transparent clip which represents animated smoke, over a landscape clip. I would like to color this smoke as rainbow using a clip being a rainbow gradient. My idea was to use the luma keyer on the smoke and to take the "keyed pixels" from the rainbow gradient clip, but unsure how to do that. Or is there another way / plugin ?

In final cut, you can place the gradient in a "layer" under the keyed clip. Set the Blend Mode of the keyed clip to Stencil Alpha. Group the two into a CC if you need to overlay the colored effect on another clip. (Stencil Alpha will "cut through" *all* layers of clips in the storyline, but is isolated inside a CC).