Other than using a color mask, meaning defining the red color and then reducing its saturation to 0—which is exactly what a photo app does, too—I don't see how. But on a
moving picture? There's
no way I see that happening with just the standard FC tools in any usable way. It's simply not what they're are geared for. Because that red will also be in the person's face, so if it's not 100% precise it should get pretty ugly.
You're in need of much more sophisticated trackers such as the one in Resolve or mocha etc.
The closest you'll get
for free and
within FCP is most likely the Boris Continuum FCPX Looks filter
The free Continuum FCP Looks filter is a powerful plugin that includes presets for color gradients, diffusion, lighting gels, film stocks & lab processes!
vfx.borisfx.com
which in fact has the mocha tracker in it. Whether it'll do exactly what you're looking for… no clue. But a very worthwhile filter to have either way.
A rough explainer video also showing the tracker (ironically with just the right thumbnail 😄):
But in the end that is a
very tall order and I wouldn't get my hopes up!
