How can I change one specific color in a photo to another color?
Asked 1/2/2016
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What’s the best way to remap colors in a photo—such as orange skin to pink, green leaves to yellow, blue sky to cyan, or a dark road to violet/blue? I’m especially wondering whether this can be done with Curves or Levels, and whether there are fixed RGB values for turning one color into another. What tools or workflow are best for this kind of color grading?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The basic answer is that you can only modify with curves (do not use levels) If and only if you do have the primary color component on the mix, and you can only shift the colors to some degree.
You can not in this case:
Here is an example image with some clear colors.
I separated the channels and you can see for example that I only have information on the green background in the green channel. I have Zero info on the Red or Blue one. (A)
Remember your color wheel, where yellow has Red and Green components. I can move and move the red channel and nothing will happen. I can not turn that into yellow.
You can if you had aditional info
So in this case I replaced "the non info zone" with the info on the green channel. You can do this with the channel mixer: https://graphicdesign.stackexchange.com/questions/17062/how-can-i-easily-swap-colors-swap-channels-in-photoshop
Now you can remove a bit the greenish cast on that yellow simply moving in diferent ratio the red and green curves. (with the proper masking so you do not afect other areas)
It is easy to change colors simply moving curves if you have the component
orange skin tone to pink
Add more red or remove the green and in less proportion the blue
green leaves to yellow
You can not directly as the previous example.
blue sky to cyan
Add more green
black road to violet/dark blue
Simply add more blue with a bit of red.
There are bettter methods
Curves is not the best way to do that.
You could on the other hand mask zones and change a Hue slider.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
10y ago
0
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There isn’t a single set of RGB values that will always turn one real-world color into another. The result depends on the original image data, brightness, and how much red, green, and blue information is already present.
Curves can help with limited shifts, but only if the target color can be formed from channel information that already exists. Levels is less suitable for this kind of targeted color remapping. For example, if an area contains only strong green information, simply moving RGB curves may not be enough to turn it into a very different hue unless you add or remix channel data.
For most practical color changes, a Hue/Saturation adjustment is usually the easiest tool because it lets you target a color range and shift it around the color wheel more directly.
If you want larger or more flexible color transformations, LAB color can be better than RGB because it separates lightness from color, making stronger hue changes easier with curves or channel blending.
In short: use Curves for subtle channel-based shifts, Hue/Saturation for straightforward color replacement, and LAB for bigger or more precise color remapping.
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