Best way is first place your controlpoint to the spot you like, then switch to maskingmode, the black and white screen, black is not effected and white is most effected. Move the “eye” , it looks like a eye, with a iris and a pupil, that way that the pupil is on the exact color/place you like to change.
Then switch back to normal screen and desaturate 100% you see by part of you image turning grey how much it effects the parts in the image. (i did a example in the tutorial section)
One example there are more.
Note the circle is not a border its a effectarea increse decrese, if you don’t want a piece not be effected use negative controlpoint to block the effect.
Regards peter