Creating color luminosity masks

[Hraw]

Hi guys i would like to make color selections from the channel as it happens for luminosity masks. This time instead of select RGB i need to select a single color. I would like to know if it is possible to create by alpha channels selections for Red, Green, Blue, Cyan, Magenta, Yellow and Orange. Let me know please

Thank you

[c.pfaffenbichler]

You could work in Lab.

There you might even be able to use Blend if settings of Smart Object instances as the basis of a Clipping Mask.

[Hraw]

if I work in lab color in the channels I should find cmyk right? Once I have selected and created a new level with that alpha channel when I need to convert them back to RGB I have to make a flatten. There is something more versatile is it possible?

[c.pfaffenbichler]

if I work in lab color in the channels I should find cmyk right?

I am not sure I understand what you mean.

Once I have selected and created a new level with that alpha channel when I need to convert them back to RGB I have to make a flatten.

One should avoid performing needless conversions.

If you indeed want to create a Layer Mask or Alpha Channel rather use a Lab duplicate than damaging the original image by converting it to Lab and back.

[Hraw]

Don't know how to use a LAB duplicate sorry, the only way i know to switch lab is by mode and convert in lab

whether I would  to stay on red green and blue only, how many alpha channels I can create for each color?

[c.pfaffenbichler]

Don't know how to use a LAB duplicate

Image > Duplicate, then convert that copy to Lab and copy/paste, do the necessary operations and duplicate whichever Channels you want back to the original file.

But maybe this is a bit too complicated for your needs.

Have you tried using Select > Colour Range (in the original RGB-image)?

[Hraw]

My intent was to create actions to easily select these areas and fasten some operations, then as I pick that areas is not important, I only need accurancy

[c.pfaffenbichler]

I only recommended working in Lab because there, unlike the R-, G-, B-Channels in RGB and the C-, M- and Y- Channels in CMYK, the a- and b-Channels (theoretically) carry no meaning with regard to Luminance, so it could be somewhat easier to isolate regions of certain hue and/or chroma in that Colour Mode.

[c.pfaffenbichler]

[Chuck Uebele]

When I need something like this in L*A*B*, I make a duplicate file and convert that, then import my mask or whatever back into the RGB version, thus avoiding needless color conversions.

[Bob_Hallam · Accepted solution]

Going to Lab color for any reason requires a high bit depth or you risk image degradation.  16 bits minimum are needed to keep this from happening. 

To create your luminosity masks all you need to do is create duplicate image layers with the channels required for the mask and select create a selection based on those combined channels.  For example, to isolate luminosity in an RGB image, there's no need to convert to LAB, simply Command-click (Mac) the RGB channel to make it a selection.  This selects the luminosity. 

So to take that method and answer your question, to create the luminosity of cyan, you need the Red channel added to the Blue channel which creates cyan then Command-Click for its luminosity.   Set Non-linear History in the History palette, Then Using this same method you can separate out each color until you have what you need.   and fill from history to get back to the orig image.