Photography BLOG

How to correct your color using RGB values
by Chris Nicholson

Part I — Part II
Part I: Prep Work

Several times people have asked me, “How can you be a photographer if you’re color blind?”

This question comes after these people learn that my eyes are particularly insensitive to the red/green areas of the spectrum. Sure, I can tell the difference between a red flower and green lily pads; I see red and green, just not the same way most of the human race does.

Still, color perception has nothing to do with how you capture an image through a lens; you see what you see, I see what I see, and if we know how to make a good exposure, then everyone’s happy.

The only problem with a colorblind photographer comes in making a print. When I was in college I knew I could never be a printer in a lab because I wouldn’t be able to accurately color-correct an image.

Alas, in the digital age, that’s no longer true. Anyone who knows how to read numbers can color-correct an image with a computer. Here’s how.

It’s a black, white and gray issue

Almost every image will have a black point and a white point — that is, a place in the photo that should be absolutely black (usually deep in a shadow) and another that should be pure white (a bright cloud, water rapids, etc.). You need to find those two points and determine the RGB value of them.

First, do a quick Levels adjustment. Move the sliders in to just barely clip the ends of the black and white ends of the histogram. Some people like to clip off more information in Levels, but I don’t like to blindly throw info away; if I want to throw out more info later, then I can do it then.

Next you need to find where in the image your black and white points are. An easy way to do this in Photoshop is to navigate through the menus to Image-Adjust-Threshold. Move the slider to the far left to see the black point, to the far right to determine the white. (You may need to nudge the slider back in from the edge to see the points.)

Cancel out of the Threshold dialog and position the mouse pointer over your black and white points. Read the RGB value for each point in the Info box, and write the values down. (Alternatively, with the Eyedropper Tool, you can shift-click on each point, which will lock the values into the Info box.)

Next, try to find something in the photo that should be gray – snow in shadow, faded cement, a gray shirt, etc. Now record the RGB value for this gray point.

(The gray point can be a lot harder to find than the black and white points – in fact, in some cases, there simply may not be any pure-gray elements of an image. In these cases, just use the black and white points.)

Part II: Correct the Color
Reset the RGB

Using Curves, you want to make the black point pure black and the white point (almost) pure white.

Open the Curves dialog (or, even better, define a Curves layer, if your image-editing software allows). What you want to do to the curve is move your earlier reading for the black point and force it to an RGB value of 0-0-0. Go to the Curve for Red and click on the black point (zero). Set your Input point as the Red reading that you recorded earlier, and leave the Output value as 0. Then do the same thing for the Green and Blue values.

By equalizing those Red, Green and Blue black-point values, you’ve now removed any color-cast at the black end of your Curve, and you’ve set the blackest point of your image to absolute black.

Next, do the same thing for your white point, except we don’t want it to be perfectly white. Pure white — i.e. 255-255-255 on the RGB scale — results in no ink being laid on the paper, which doesn’t look natural. To get some ink, but not so much that the area looks gray, we want to set the RGB value of white point to 240-240-240.

So for the white point of your image, go to the Red, Green and Blue Curves, set the Input to the values you recorded earlier, and the Output to 240. This removes any color-cast at the white end of your Curve, and produces a white that will look natural when printed.

The Gray Matter

We handle the gray point a little differently. To remove a color-cast completely, we want to set the RGB values of the gray point so they’re all the same (for example, 115-115-115; when all three RGB values are equal, that produces black, white or a shade of gray). This isn’t as straightforward as setting the black or white points.

The four most common corrections for the gray point are as follows:

1. You want the average. If the RGB values of the gray point are somewhat equally scattered (i.e., 100-116-128), calculate the average. (In this case, the average is 115.) In each of the Red, Green and Blue Curves, click in the middle of the Curve to define a point, then set the Input value to what you recorded earlier as your gray point, and your Output value to the average value that you calculated.

2. Only one color is off. If two of your RGB values are about the same but the third is way off, then you probably want to just rein in that renegade color. For example, if your gray point’s RGB value is 100-100-160, then you’re looking at a photo with a serious blue cast. In this case, try 100-100-100, or perhaps 110-110-110; try a few things to see what looks natural.

3. Why are you messing around with that color? Sometimes you don’t want to remove a color cast. A photo of early-morning mist whispering over a prairie may look better with a blue cast; a photo of late-day light falling through a canyon will certainly look better with a red cast. Your best option may be to leave the gray point alone. Or to adjust it just a little; in the previous example, adjusting that 160 Blue value to only 130 may be the perfect solution.

Gray-point is not an exact science. After color-correcting a bunch of photos, you’ll learn how to adjust your gray point, and when not to bother even trying. The point is that you learn to recognize a color-cast by looking at the RGB values rather than relying on your eye. (200-100-200 indicates a red color-cast; 100-200-100, green; and 100-100-200, blue.)

Clean Things Up

All this color-correction has likely wreaked havoc with your photo’s brightness and contrast. Use your preferred method of adjusting for those problems. I generally use the collective RGB Curve, and then I’m done. And I can go brag to my friends that I, Colorblind Chris, color-corrected a photograph.

Notes

• When setting your white point, be sure to select something that is actually white. Often a specular highlight, such as the sun shimmering through a raindrop or a bright light reflecting off metal, will show up as the whitest point in a photo, but it’s not the pure-white spot you’re looking for. Pick something that’s truly white and let the specular highlights blow out — that’s how they look best anyway.

• Your photos won’t always contain an element that’s pure gray. What to do in that situation? Eyeball it. Being red/green colorblind, this is when I usually defer to someone else’s opinion. Sisters are good for this. Another suggestion is to get a non-color-blind spouse.

• This color-correction method also works in CMYK mode; just adjust accordingly.