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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.

Texture with icons

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:

Transmission map

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

Node setup

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.

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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.

enter image description here

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    $\begingroup$ Oh so you select it not based on colour but based on its BW contrast with respect to the rest of the texture? $\endgroup$ – Fiztban Jul 26 '20 at 22:28
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    $\begingroup$ Yes. In this case it works, because there are just two colors. To isolate a specific color from a photo, I'd reccomend this answer by Gorgious. $\endgroup$ – Jachym Michal Jul 26 '20 at 22:30
  • $\begingroup$ Not sure I am able to use the added answer you provided in the comments. I make the node connection and use the Combine RGB to specify the Red colour from the texture and it just makes everything transparent. I must be doing something wrong $\endgroup$ – Fiztban Jul 26 '20 at 22:52
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    $\begingroup$ Actually what worked was the Separate RGB node and maths greater than 0.95 for the red channel. That did the trick for this case. Thank you again! $\endgroup$ – Fiztban Jul 26 '20 at 22:58
  • $\begingroup$ No problem :). If you unplug the Separate RGB node, you can simply choose a color to isolate :). $\endgroup$ – Jachym Michal Jul 26 '20 at 23:01

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