How to use a texture colour to define a transmission map I have a model that has a texture applied to the plane. The red icons are actually supposed to light up because of an LED behind them.

To achieve a controllable transmission level on my map I took the texture and isolated the red icons as pure white on a black background:

And then plugged this into the Principled BSDF along with a Mix RGB node to control the level of transmission:

Could this be done by simply taking the original texture and isolating the red colour and defining that as my input using nodes?
This would be helpful in future in case the texture changes, then I wouldn't have to re-do the transmission map as well.
Sure thing.
Just add a ColorRamp and increase the contrast.
You can also set it to Constant (for maximum contrast) if you don't mind jagged edges.