hugopics wrote:
Unlike Lab, it seems that U/V channels separate warm and cold colors,

U wold be blue channel and V would be red channel with negative values yellow and cyan. Not exactly warm and cool. Depends on negative value.

PS. No, you are right. Negative value would make it black. It is warm and cool. Just to make it completely warm and cool mask you need to invert one of the channels and to mix U and V in lighten or darken mode. And it would be chromatic warm and cool, without luminosity.

hugopics wrote:
thanks for your infos Tulac ! useful…

But finally I find easier to make a cool and a warm masks directly in Lab.

You can use warm and cold luts. Would be much faster.