This is actually super simple and you don’t need rgb primaries for it.
Can be done with any module that influences brightness.
The idea is to gradually darken the vibrant colors so that an additional color contrast is created. This restores the depth of detail lost by the strong saturation.
In this case I will use the color balance rgb module because you can additionally enhance the effect with brilliance sliders.
This is what the photo looks like and the effect will be visible on the right side:
First we create a saturation gradient mask with the emphasis on the most saturated area…
…and additionally a brightness gradient mask with the emphasis on the darker areas:
How strong the gradient should be can be decided according to the motif
Now we have such a mask:
Now you can use the global brilliance slider to darken the masked area and thus achieve the desired contrast:
You can also play with other sliders in the perceptual brilliance grading to increase the contrast even more:
The blue version of the effect works on the same principle. You must additionally restrict the mask to blue.