Garder une couleur en N&B

One ‘old’ method is to use Saturation sliders in the Color Mixer to affect colors globally. You can use masking to be more precise with selected areas. (eg. Select ‘Subject’ or a ‘Color Range’, invert mask, desaturate.)

If you convert the image to B&W by toggling the [B&W] button and use a mask, you will only have a ‘Tint’ option in the Color panel.