Selecting pixels by specification in Photoshop

I do not have a complete solution, but I do have some ideas. Getting each of the colours into their own channels should make things easier to interrogate.

You can do this by selecting a pixel or two of a colour (and only that colour), then choose Select → Similar.

With all the pixels of a single colour selected, you can now save the selection, which will create a new channel.

Here’s where things get a bit more difficult and hand-wavy. Through some processing, we should hopefully be able to keep the lines that are 10px wide or longer, but remove all others.

Here’s a test pure black and white selection with some varying line lengths. The top two lines are 12px and 10px wide respectively, and the ones underneath are shorter. We’re trying to keep the 10 and 12px lines, but remove the others.

Duplicating the selection a few times, offsetting it, and using varying opacity for the layers gets us part of the way there. I used 5px offsets and 50% and 33% opacity, plus levels to take the grey pixels back to white (keeping the ones we want).

Another pass using the same method, but this time with a 3px offset.

And a final pass, doing exactly what we did in the first pass. I think this should give the required result, and also be able to be automated by recording an action.

With a good selection, filling those parts of the image with a pattern should be pretty easy. A pattern layer using the newly created mask would work for that.

Here’s a GIF of the steps used. I think it should work with any initial artwork.

Now I think about it, I think this entire structure could be set up using smart objects and smart filters, so you’d just have to replace the contents of the smart object to process it (assuming the source artwork is the same dimensions).