A little about why I love L*a*b (Lab) colorspace.
The introduction I had to L*a*b colorspace came one summer when someone recommended Dan Margulis’ book Photoshop Lab Color, to me. It became evident to everyone around that if I wasn’t a photogeek before this, I was afterwards! I devoured that and every other book I could find on the subject and started an ongoing love affair with using L*a*b as my primary working space. (Though I do admit to adopting a few others in recent years- ProPhoto is quite nice for the ranges and eciRGB v2 has some interesting qualities ).
For those of you who have little to no practical knowledge of this colorspace, I’m including a mere introduction here. For further and certainly more in depth understanding I recommend the following texts: Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace, Professional Photoshop: The Classic Guide to Color Correction both by Dan Margulis. Photoshop CS3 Mastering Lab Color by Deke McClelland.
Meanwhile for a brief introduction read on.
Lab is designed to approximate they way we humans see. Unlike RGB and CMYK which were designed for other mechanisms, i.e. electronic systems such as computers , televisions and printing presses, Lab attempts to map colours they same way we perceive them, though many of the “colors” within Lab space fall outside the range of human vision, and are therefore purely imaginary. The gamut is based on three axis, L= Luminosity or light/darkness, A*= magenta/green range, B*= yellow/blue range.
What I like about this setup is that it’s similar to how a painter thinks about colours. As an example; when I’m painting I look at the subject and ask myself questions about the colours. Is the colour on the cheek light or dark? Is it warm or cool? Is it more green or more blue? More yellowish or more purple-ish? In a similar way we can use Lab to answer questions about what colour something is, and better still…. What colour do we want it to be? This makes colour correction by the numbers a really simple thing to learn to do!
Rather than deal with copyright issues use this link to jump to some wonderful information specifically about Lab. For a really great graphic of the Lab gamut click HERE to jump to Bruce Lindbloom’s site. After the jump, click on the INFO tab, then scroll down to the Lab Gamut Display.
For a less robust description here it is: Imagine a crossing of two lines with a third line sticking up like a mast. That third line represents light at the top (white) and dark at the bottom (black). So “A” represents the “either magenta or green” balance and “B” represents the “either yellow or blue” balance. This also means that the luminosity or brightness is totally independent of the colour! There are some of you at this point who may be jumping up at down at the freedom this brings! It means that we can change the colours without making things lighter or darker (as we are forced to do in both RGB and CMYK colorspaces). Whooppee!
Just as in mixing oil paints, if you mix equal amounts of complementary colours you produce a neutral (white, grey or black). If you have more of one complementary colour, you’ll have less of the other.
The Lab scale reads like that positive and negative numbers ruler we all had in grade school. (Well at least we did when I was a kid). It reads from –128 through 0 up to 127 for the A or B channels and from 0 to 100 for the L line. The A channel goes from the greenest green (A = -128) to the most magenta possible (A = 127). In between, it passes though A = 0 where the colour is neutral. The B channel goes from bluest blue (B = -128) to the most yellow (B = 127). When B = 0, the color is again a neutral. 
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So in short terms, minus numbers mean we have a cool colour, positive numbers mean we have a warm colour. The higher the number in any direction= the more intense the hue.
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Let’s look at a practical application of colour correction using Lab. Here’s a typical landscape scene. We’ve got green trees, blue sky and white/greenish rushing water.
If we look at the info panel in Photoshop®
We see the colour numbers in Lab for the trees (31,-7,18) a darkish green yellow, the sky(35,-4,-31) a darkish hint of green, very blue and the water in the rapids area (87,-2,1) a very light green blue.
Now if we change the balance in the trees simply by changing the numbers, we get a very interesting if unreal effect.
The new numbers here are L=31, a=17, b=18. an “autumnal” scene where the trees (and the water) have shifted towards magenta and yellow.
If we liked the trees this way we could leave them and correct the water and sky back towards their natural colours. (sky is now 37,0,-38) a darkish, neutral, blue).
Why I find all this exciting is because we can change the colours independently of the luminosity, which means there are all sorts of cool corrections, sharpening, and experimentation that can happen as you really start to play within the colourspace. Think of Photoshop CS4 and the ability to separate channels out and recombine them to create new files. What can be done if the channels are Lab instead of RGB or CMYK!
-Amy Kosh
For more interesting links related to working with Lab use the following:
Nikondigital.org
Bruce Lindbloom
Dan Margulis
Adobe
Deke Mcclelland