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



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?
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.