How do I change all the colours in an image to white, except for the black?
Is there any shortcut to do it?
For example, in the next image only the black parts would remain.
Like anything in Photoshop there are many ways to do it. I believe this way has the best speed/result ratio; it takes a few minutes and retains most of the details of the original image:
1- Use the burn tool (Range=Shadows, Exposure=100% and Protect Tones checked) to darken the black lines even more, especially on the dog as this black contains a lot of yellow. You can pass quickly all over the image, but avoid the man's blue shirt.
You should get a result similar to this:
2- Use Image / Adjustments / Selective Color (or use an Adjustment layer) and lower the cyan, magenta, yellow and black of every color except the blacks to -100%. You should get something similar to this:
3- Use Image / Adjustments / Curves (or use an Adjustment layer) to darken the shadows and lighten the quarter tones.
With this curve:
I obtained this result: