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One of the most powerful Photoshop tools at your disposal is Curves. Though it’s often only used to tweak contrast, the curves tool is also hugely effective in correcting color. What’s more, learning how to use it gives you a greater knowledge of image editing in general. It’ll enhance your understanding of the histogram and teach you how to edit photos by numbers.
Correcting color in images is about removing unwanted color casts. The “unwanted” part is important because some color casts are desirable. For instance, you wouldn’t want to neutralize the warm hue of a sunset. However, you might want to remove the blue color cast that sometimes pervades photos taken on overcast days or in hazy conditions. By removing an unwanted color cast, you’ll reveal the true color of the objects and subjects in your photo and make the image “pop”.
The content of a photo will dictate how you edit it, so you shouldn’t obsess over correcting color in every photo. Many times, you’ll want to do little or nothing to the color. An appreciation of the curves tool and the numbers around it will help you decide what each photo needs.
When working with curves, histograms, and RGB numbers, it’s useful to know what the histogram is telling you. There’s no such thing as a right or wrong histogram, per se, since it only mirrors the pixel data of the image, but it will highlight potential problems.
By looking at all three RGB (red, green and blue) histograms at once, you can immediately get an idea of whether or not the image has a color cast. If there’s no color cast, the three histograms will look very similar. A black and white RGB image illustrates this perfectly because it’s completely neutral. In that case, the three RGB values will be equal in every part of the image and the histograms are identical.
Before you start in curves, there are a couple of things you’ll need to prepare in Photoshop:
Whenever a photo contains an area that should be neutral gray in your estimation, you can use the mid-tone eyedropper tool in either levels or curves to quickly correct any color cast. Simply clicking on the supposedly gray portion of the image will correct the color. It’s usually worth clicking a few times in different areas until you achieve a result that pleases you. There are ways of calculating precise mid-tones in an image to make this method more precise, but guessing often works well and is far quicker.
By introducing the info palette into the equation, you can make far more precise color corrections. The technique you’re about to learn also teaches you to evaluate and edit photos by the numbers. Think about this – when you have nothing to compare an image to—no alternative version—it often looks “okay” at first glance. By studying the RGB values, you’ll get a clear idea of any potential problems in the photo.
Before you proceed, it’s important to note that an image always needs “neutral” areas for color correction to succeed. That’s because a neutral tone provides a known reference point that you can work from. Neutral pixels always have identical RGB values (e.g. 128, 128, 128). Any photo that doesn’t contain a neutral tone is difficult to accurately correct. This is true whether you’re adjusting color yourself or hitting an auto-color-correct button. Photographers often use gray cards to introduce a known neutral into the image for color correction later.
Here are the steps you might take to correct color using curves, the info panel, and histograms:
With your image open, select the eyedropper tool from the Photoshop tools palette.
Hover the eyedropper tool over a diffuse white highlight in the photo with RGB values in the 230s or 240s (try to avoid high 250 values). Use the info palette to see these values.
Hold down the Shift key and click to create a sample point from this white area, which will show in the info palette as #1. It’s possible to move a sample point after you’ve created it by holding down the Shift key and dragging.
Repeat this procedure with a neutral gray mid-tone, if you can find one, with RGB values of around 120-140.
Do the same thing with any black, shadow areas with values of about 10-30. After that, you’ll have created three sample points. Since color casts in shadows are inherently harder to see, this third sample point can often be skipped without ill effect.
Looking at the three RGB samples you’ve created, you should get an idea of any color casts that are present. You’ll typically see the same problem across all tones from highlights to shadows, though not always. Remember that a low RGB value in any of the three channels indicates an opposite color cast. Thus, a low red value indicates a cyan cast, low green is magenta, and low blue is yellow. This only applies in areas that should be neutral in color (i.e. white, gray, black).
Open a curves adjustment layer. Hold down the Ctrl and Shift (Cmd + Shift) keys and click once again exactly on the center of the second, mid-tone sample point you created (#2). This has the effect of placing a mid-tone point along each of the individual RGB curves.
Now it’s time to correct the color cast. On a curves graph, the top right point represents highlights and the lower left shadows. In between are any mid-tone points that you placed on the curve.
Starting with highlights (your #1 sample), open the individual red, green and blue curves channels one at a time and move the top right point either left or down along the outer edge of the graph so that, eventually, the three values match. As you move each point on the graph, the info palette gives you the updated output value.
Usually, it’s best to choose the lowest or middle of the three existing highlight values and match the other two to that (see “tip” below).
Repeat this process with the mid-tone and shadow points, so that all of the chosen neutral points in the image are in fact neutral. The bottom-left shadow point is also moved along the outer edge of the graph, either upwards or right. The mid-tone point you’ll drag either up or down. If the color looks wayward at the end of this process, it typically means that you’ve picked a sample point that wasn’t neutral. Ensure that your sample points contain no color noise or reflected color. Zoom in on the area you sample to make certain of this.
Now all points are roughly similar, in other words, all the sample points I took that were estimated to be neutral have been made neutral. Note that the numbers don’t have to match perfectly like this as long as they’re close. In the curves graph, the red channel has been lifted and the blue channel pulled down slightly as a result of my edits. The green channel was untouched in this instance, so the corresponding line cuts straight through the middle.
Once the correction is complete, the sample points are removable by holding down Ctrl + Alt (Cmd + Option) keys and clicking on them. You should see the scissor icon when you hold these keys down. To finish, either save the image with its adjustment layer intact or flatten the layers, as required.
Tip: Since moving the endpoints of the curve line affects all highlights and shadows, you should edit conservatively. In particular, avoid choosing the highest of the three RGB values as the target when matching red, green and blue highlight channels. Otherwise, you may find that you blow out wanted detail in the brightest part of the image. Flaws in the shadows are generally less noticeable, but you still risk blocking detail if you adjust all the shadow RGB points to the lowest of the three values. In general, turn the numbers away from their extremes.
The types of correction discussed in this article work best when there are naturally occurring color casts in the image. In mixed lighting, where the light sources are radically different (e.g. incandescent lighting and daylight), you’ll need to painstakingly address each affected area of the image using layers in Photoshop or the adjustment brush in Lightroom. Avoid this type of lighting wherever possible, since it’s difficult and time-consuming to correct in processing.
I don’t expect that you’ll use these techniques on every image, but I hope they’ll improve some of your pictures and that you’ll enjoy experimenting using curves in Photoshop. This type of mathematical editing gives you a good understanding of histograms and the meaning of RGB values.
Merely hovering the eyedropper tool over a picture while watching the numbers will tell you something about it. If there are no naturally occurring “neutrals” in the photo and you want consistent or accurate color, a high-quality gray card provides a solution.
Please don’t hesitate to fire questions my way if anything is unclear.