How to remove color areas from an image?

Question

How I can remove all color areas from an image?

I don't mean desaturate. Desaturate turns color into gray. I want to turn any color into white, and preserve any existing grays and blacks.

For instance in old print engravings they would dab paint onto the engraving print. I would like to restore it to the original engraving part of it and remove the painted on parts. Or something similar with old pen & ink illustration when they would start with a black and white image and then a colorist would screen a few solid color areas on top of it later, I would like to be able to restore it to the original colorless drawing.

Example of an engraving, I would like to remove the red splotch and restore it to the original engraving:

Answer 1

I am not certain this will work for every image, if there are many. However, this works well for a color here or there.

• Convert to L·A·B mode to isolate color from values
  • If there are varying colors in the same image, convert to L·A·B then make a selection before using Hue & Saturation.
• Choose Image > Adjustments > Hue & Saturation from the menu
• Select the color you wish to eliminate (red)
• Move the Lightness slider all the way to the Right
• Move the Saturation slider all the way to the left.
• Color should be gone
• You can then convert back to RGB mode if necessary

This method tends to retain all the subtle values (grey splotches) a bit better, without boosting contrast of the image overall and merely eliminates color.

Answer 2

Using the Channel Mixer (Window → Adjustments) and selecting Monochrome and then setting everything all the way up...

...produced somewhat desirable effects:

Though, it did result in a bit of a loss of detail and it couldn't remove all of it.

I then messed with the Levels a bit, and got better - though not perfect, results.

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If you don't have that many images I would recommend doing this manually (or a mixture of both) for good results.

This was done in just 2 minutes of lazy work, if you actually solidly worked on it for a few minutes, you could get it really nicely.

Answer 3

This is what you can get with a non-destructive layer stack:

This method works if the color is transparent like print inks (=black below it is still black, only white is actually colored). It cannot guess, what's behind opaque colors.

There's the original duplicated into a new layer group, which at first has blending mode Pass Through (=no effect)

Over the copy there's placed a 50% grey layer with blending mode Luminosity. It shows colored areas:

The next is a Curves adjustment layer which stretches the preceding to max contrast. The curve is a nearly vertical step. It's not the same as treshold because the result is still colored:

Beware lifting too much up stray noise spots. They will make holes.

The next is Black&White mixing layer. The idea is to lift the colored area as white as possible but to keep black still black:

The layer group has blending mode ADD which lifts the levels up towards white. Mode Lighten works, too, but the result seems to be a little more grainy with it.

As you see, it's not ideal, there's still room for manual retouches. I recommend to add on top a normal layer (=pencil) where you paint some black and white. Then add a curves layer to reduce greys, if it's needed: