In my last post about correcting colour - Correcting colour: How to stop your white paper looking grey - I looked at how you can use the levels profile for the individual colours of the RGB profile to bring a very distorted colour back to its proper colour. In other words - how to stop your white paper looking grey - or, as in my example, a very vivid periwinkle blue!
These are the Adobe Photoshop & help pages which provide an overview of Levels adjustments.
You can also use the eyedropper tool in Levels to identify parts of an image which should be a true black, white or grey.
Personally I find it rather too crude - a bit like the simple quick fix adjustments which are possible in Elements.
Here's what happens when we take my photo of my pen and ink sketch in a Moleskine which was photographed in deep shade in a garden. I then applied the white eyedropper to the background paper of the image.
(left) original image (right) after the white eyedropper tool in Levels was used to click on 'blue' paper
As you can see, the pen and ink drawing has virtually disappeared whereas it came up clearly using the more longwinded adjustments from the previous post. To my mind it looks sunblasted - it's a "whiteout"!
I then magnified the image and used the black eyedropper to drop onto one of the ink lines - and this is what I got. It's a heavily pixelated version of my drawing - which isn't very evident on the blog but is IRL
Like I said - I prefer adjusting the profiles. I think I get a much more accurate colour balance that way.
However next time you produce an image of a drawing on white paper which looks grey why not try the eyedropper tool - you may have a different experience.