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In the Shader Editor, I can create an RGB node and in there, I can set an alpha value as displayed in this screenshot:

Setting alpha value in an RGB node

However, no color separation node in blender supports extracting that value and no combine node accepts it as input either.

Is there any way to separate an alpha value from a color, or to combine it into a color using only nodes in the Shader Editor?

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    $\begingroup$ Alpha is just a value, what is the need to extract it from a RGB input? $\endgroup$
    – lemon
    Commented Aug 8, 2019 at 9:16
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    $\begingroup$ If that works, you would only need one connection to transfer color and alpha value to another node. This would result in a cleaner node graph with less connections. Otherwise, you would always need to change 2 connections (color and alpha), though there are use-cases where both go along almost all the time (when working with RGBA images for example). I could separate the alpha in my custom group for example, to blend two textures and then you dont need to separate alpha inputs. That makes more sense to me because whenever I change the color input I would change the alpha value as well. $\endgroup$
    – narranoid
    Commented Aug 8, 2019 at 9:21
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    $\begingroup$ If you find that important, could try to ping the dev on this subject eventually: wiki.blender.org/wiki/User:OmarSquircleArt/GSoC2019/Proposal $\endgroup$
    – lemon
    Commented Aug 8, 2019 at 9:30
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    $\begingroup$ I'll check that out, thanks. It just felt very weird to me, that we have 4 dimensional colors passed along in the Shader Editor, but the 4th value is for no use at all. If we could have an alpha output on SeparateRGB, that could be very nice I think. $\endgroup$
    – narranoid
    Commented Aug 8, 2019 at 9:35
  • $\begingroup$ Second this - seems very weird to me that the compositor has Separate RGBA where the shader has Separate RGB $\endgroup$
    – Oliver
    Commented May 18, 2020 at 6:00

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It's only a partial answer to the body of the question and it's a bit hacky. You can use a ColorRamp node, set the Factor to 0 and tweak the color and alpha values of the first stop. This would effectively emulate a RGB node with an alpha output.

But you you can't use it to combine RGB values and an Apha value since the color ramp node doesn't take inputs for its color stops.

enter image description here

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