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This might work.

Scan the plans at it's native resolution, look at the documentation for that.

You need a high quality scan.

Assuming you have red, blue, and black, open a curves adjustment layer under Presets click on "Sample image to set for white point".

Make sure you're zoomed in and sample size is set to point sample.

When you sample red it should disappear because you set red to white.

Open another curves layer and do that for every other color.

It works on a test that I did with a few lines on a white background not sure how well on your scan but it's worth a try.

Good luck.

Posting part of the scan like Tom said would help.