how can i separate colors from light ?

[mantra]

Originally Posted by Der_W

This is because in RGB luminosity and color is not as easily separated as in say LAB where there is an extra Lightness channel.
In RGB we have to use a specific formula to extract the luminosity which is about 30% Red, 59% Green and 11% Blue (which is what happens when you just [Ctrl]-click the RGB channel - minus Gamma conversion depending on the working space grayscale profile, but we may leave that aside for now).
Now to select only the colors of your image, you'll need to subtract the luminosity from your image (e.g. instead of 100% in the Red channel you'll have about 70% Red, -59% Green and -11% Blue).
Because this returns in RGB values < 0, you might want to add an offset of 128.

but is there a way to select all the luminocity?
i mean how can i do it using your formula

In RGB we have to use a specific formula to extract the luminosity which is about 30% Red, 59% Green and 11% Blue

[Der_W]

One easy (and accurate) way would be adding a new 50% gray layer on top of your stack and setting its blend mode to "Color". Now you're left with luminosity information only and use it however you like it.

[mantra]

Originally Posted by Der_W

One easy (and accurate) way would be adding a new 50% gray layer on top of your stack and setting its blend mode to "Color". Now you're left with luminosity information only and use it however you like it.

thanks Jonas Happy B.!

and this should work?

1) channel palette->ctrl + click on rgb -> save selection (lights)
i can see in the histogram the lights selected

2)select->inverse ->save (shadow+midtones)
now i can see in the histogram shadow and midtones

3) ctrl click on lights+ctrl+shift click on (shadow+midtones) -> save (all light)

adding lights + shadows and mitones =all lights ,i can see in the histogram

[Der_W]

To select just the lights (with a smooth falloff to the shadows), you can just [Ctrl]-click on the RGB channel, that should work.
Inversing the selection and adding the original selection to it should be a selection of the whole image (at least with a gamma 1.0 tone response curve, will probably be a little different with other gamma curves, however I can't test that right now). So no need to save the inverse as well as long for selecting the highlights .
(Maybe if you don't understand the explanation above this will help: Selecting highlights selects x amount of pixels. Now selecting the inverse will get you MAX - x. Adding the original x pixels to it again will be MAX - x + x = MAX, so you get all the pixels in the whole image. Hope that clears things up a little and doesn't add to the confusion ).