Did you ever wonder what Earth would look like with a red Martian sky? Or if a cerulean rose would smell — I mean, look — as sweet? Maybe you just want to see how cool you’d look with the golden eyes of a cat (spoiler alert: it doesn’t really look good on me, but maybe it’ll work for you).
Like so many things in GIMP, there are a lot of different ways you can adjust the colors in your image. Every editor has their own favorite way, but let’s take a closer look at some of the simplest ways you can replace one color with another in GIMP.
There are two basic approaches, and choosing the “best” one depends on the contents of the image you’re editing. You can directly select a specific color and replace it, which works best on simple images, or you can choose a range of hues and adjust all of them relative to the rest of the image.
The Quick Guide to Changing Colors in GIMP
This isn’t always the best way for every image, but here’s the quickest way to change one color into another in GIMP:
- Step 1: Use the Select by Color tool to select all the pixels containing the color you want to change.
- Step 2: Open the Colors menu, select the Hue/Chroma filter, adjust the Hue slider until you’re satisfied with the results, and then click OK.
- Step 3: Pat yourself on the back, because you’re already done!
This guide is intended for GIMP users who are already fully familiar with the ins and outs of the program, but I’ll explain the steps in more detail down below for those of you who want a bit more detail, along with a technique for creating more gentle color shifts for complex images like photographs.
Replacing Simple Solid Colors
This method works best on a relatively simple image that only has a few colors, such as a GIF or other image that has a limited color palette. When I say a few colors, I mean from a numerical value perspective: each color should be consistent across the whole image to get the best results.
Pixel art, logos, and outlined text are great examples that are perfect for this technique, but it won’t work on pictures with too many aliased pixels. Here’s how you can apply it:
Step 1: Create Your Selection
The first step in the color change process is selecting the pixels that you want to modify. There are a couple of different ways you can do this, but they’re both found in the same spot in the toolbox: the Select by Color tool and the Fuzzy Select tool. They work almost identically, except Fuzzy Select only selects contiguous areas of color while Select by Color can create multiple selection areas at once.
Step 2: Choose Your
There are numerous filters dedicated to color adjustment in GIMP: Hue/Chroma, Hue/Saturation, Color Exchange, and Colorize, to name just a few. Each one has its strengths and weaknesses, but it’s hard to beat the Hue/Chroma filter for pure simplicity.
Open the Colors menu, and select Hue/Chroma from the list of options.
As you can see in the screenshot below, it offers you three sliders for adjusting your selected pixels: Hue, Chroma, and Lightness, often referred to as the HCL or Lch color model in GIMP. It’s an alternative way of representing the RGB color space used in most digital images, and it’s capable of describing all the same colors.
The Hue slider lets you set the color, the Chroma slider sets the intensity of the color between 100% grey and 100% color, and Lightness determines (you guessed it) how bright or dark the color is. It can take a second to get used to thinking about color in this way, but it works well once you’re familiar with it.
Pushing the Hue slider up to 120 or so gives Wilber a fetching teal paintbrush! So much nicer than the orange/brown from before.
Step 3: Recycle Jokes
There is no Step 3 – you’re already done!
Hue Adjustments in Complex Scenes
When you’re editing more complex images like digital photos, it’s a bit more unusual to get large sections of the same color next to each other.
Subtle variations in color tone can make it very hard to get good results with the Select By Color tool, and it looks more natural to have gradual color transitions in a photo. Sharp lines look great in a logo, but they really stand out in a photo.
You could spend your time with the Select By Color tool and then feather your selection edges to create a softer transition, but it’s easier to use the Hue/Saturation filter to adjust a whole range of similar hues all at once. It’s a bit tricky, but here’s how it works:
Open the Colors menu and select Hue/Saturation from the list. GIMP will open the Hue/Saturation dialog box so you can configure your adjustment.
If this dialog makes no sense to you at all, don’t worry – I’ll explain. It’s probably one of the most counter-intuitive implementations of a Hue/Saturation adjustment filter I’ve ever seen, largely because it copies the circular hue map that you might have seen in other programs, but without actually showing it to you in all its detail.
You can adjust the HSL values of the entire scene as a whole by selecting the Master button in the middle of the color wheel, or you can choose to focus on a specific section of the color wheel by selecting the appropriate color.
Since most photos don’t contain just these six pure primary colors, the Overlap slider allows you to expand your adjustments. If you want to adjust orange tones in your image, you can select either the Red or Yellow option and adjust the Overlap slider until you’re affecting the desired tones when you adjust the Hue slider.
The Hue slider setting ranges from -180 to +180, covering the full 360 degrees of a color wheel. You’ll probably notice that setting the master to -180 results in the same color change as you’ll get from +180 because there are only 360 possible degrees in the color wheel.
As you adjust your Hue slider, you’ll see the affected colors change in the Hue/Saturation dialog box, as you can see in the screenshot above. The Red option at the top has shifted to some kind of horrible chartreuse like a neon Kermit the frog, and all the others have shifted just as far.
Of course, I’m pushing the adjustments to the extreme in order to make them as clear as possible, but most of the time, you’ll get the best results when you keep your adjustments fairly close to the original. The greater the adjustment, the more care you need to take in order to keep everything looking realistic!
A Final Word on Digital Color
The technical aspects of color models and color theory are often overlooked by new image editors, but if you really want to make yourself a better digital artist, you’ll need to at least understand the basics. I know that it’s much more fun to spend your time editing, but a bit of grounding in the theory can really help.
If you want to do some extra credit reading about color models, there’s an excellently exhaustive collection of color resources available compiled by David Briggs over the span of many years. Some of the content is extremely advanced, but there is lots of good info for beginners in the mix too.
That’s everything you need to know about how to change one color to another in GIMP – and a little bit extra on top for those of you who are dedicated students.
Do you have a different method for adjusting colors in GIMP that you prefer? Let us know in the comments, and I’ll take a closer look.About Thomas Boldt