Color Correction Basics
By Nick Ustinov.

Professional color correction has always been one of the most important jobs in prepress business. There are some minor differences between a well-scanned picture and the same picture after color correction.

Using Photoshop we can achieve excellent colors even on a photo that had been scanned with old or non-calibrated scanner. Anyone can look at the picture and say that there's something wrong with it, but not everyone can see what exactly is wrong. In order to color-correct it you must know what's wrong with the picture before color-correcting it. Here are some fundamental color correction concepts.

COLORS

First thing you should pay attention to are muddy colors - colors that look muddy or dirty. For example, to achieve good green color of a leaf you should mix cyan and yellow in varying amounts. If you introduce magenta, the leaf begins to look darker and muddier, so here the muddy color is magenta. To color-correct this problem we just remove or decrease amount of magenta.

Next thing to look at are memory colors. The sky should be blue, the grass should be green, the clouds could be white or gray. Colors you expect them to be are memory colors. So if you want your photo to look better try to correct memory colors first. Here's the example. In the left side you can see a picture with memory colors not corrected.


  

In this example you see that face in the red side looks too red that is not normal (of course she could be too tan but that's hardly ;-).

BAD CONTRAST

Another common problem is bad contrast. This means that the image lacks enough dark pixels in the shadow areas and light pixels in the highlight areas so the image looks flat. A good example would be pictures in newspapers.

So, how to fix the colors? We don't describe all methods since there's too much of them plus additional third-party plugins and filters for professionals. Here are some basic techniques. In case of bad contrast the easiest way would be Select>All, then Edit>Copy and Edit>Paste. Now you get two similar layers, one over the other.

cc2_1.jpg (9307 bytes)

cc_fig1.jpg (9796 bytes)Open Layer Palette (Window>Show Layers) and find the layer you just pasted (most probably it is Layer 1 if you didn't have any layers before.  Then change the layer apply mode to Overlay.

Woops, you got it.. The image looks way better now.. If it's got too much contrast change this layers opacity (tune it while you get the most optimal result).

 

cc_fig2.gif (7134 bytes)Another (and the most popular) technique to correct colors is using Curves. Curves can handle most of the color correction tasks you'll encounter. Select Image> Adjust> Curves to open Curves dialog box. All adjustments are made to the diagonal line. You can manipulate colors individually or all at once.

To correct only the specific colors go to Channel Palette and Shift-select required channels. Then when you open Curves dialog box only the neccessary channels will be displayed.

At the bottom of the Curves dialog box there are two very important numbers that are constantly updated as you make adjustments. These numbers represent input and output readouts of the changed pixels. By watching how these numbers change and periodically sampling actual pixels in the image while viewing the info palette readout, you should have complete control over how much color is being changed.

 

Improving contrast using Levels
In many cases you don't have to change the color to make a photo look better. You just have to improve contrast. For that reason the best tool is Levels.

cc_fig3.gif (6651 bytes)

The graphic in the middle usually resembles a mountain range is called a histogram. It gives you graphical representation of what pixels fall where in the image. If there are higher mountains of lines in the lighter portions of the histogram the image is probably lighter overall.

Colorizing with Hue/Saturation

Another popular way of improving colors (mostly used to fix up memory colors) is adjust Hue and Saturation. This is done by moving sliders in the Hue/Saturation dialog. Select Image> Adjust> Hue/Saturation to open it.

Chaning color mixtures with Color Balance

Color Balance is not a very precise color correction tool but it could be quite suitable in many cases. It lets you change balance between different colors. For example you can decrease amount of Reds and add Cyans. What's nice about this control is that you can target just shadows, the midtones or the highlights of an image.