Color Tools

Done using GIMP 2.8 on Kubuntu Linux 18.04 LTS

The next group of tools we will look at are the Color tools. Obviously color is very important in a graphics program, and tools to add, modify, and duplicate colors are thus very important. GIMP has 10 of these tools, and 9 of them are found in the Colors menu, and also in the Tools menu. It is the same 9 tools in either place, but going to the Colors menu is one less mouse click. Interestingly, none of these 9 tools is on the Tools sidebar. If you want to have any of them there for quick use, go to Edit–>Preferences–>Toolbox, and click on the one you want to add so that you see the Eyeball icon next to it. Note that you can remove tools as well by clicking on one to see the Eyeball icon disappear.

So we have covered the 9 that are on the Colors menu, but not the Toolbox. Then there is the one color tool that is in the Toolbox, but not on the Colors menu, and that is the Color Picker. Why this is the case I could not say, as it seems to me that Color Picker is a Color tool, but no one asked my opinion on the matter. In any case, the ten color tools are:

  • Color Picker (O)
  • Color Balance
  • Hue-Saturation
  • Colorize
  • Brightness-Contrast
  • Threshold
  • Levels
  • Curves
  • Posterize
  • Desaturate

Now the fact that the 9 tools not in the Toolbox also do not have any keyboard shortcuts should tell you that the GIMP project designers do not think you will use these tools all that often, and they may be right about that. The number one use case for GIMP (and for Photoshop, come to that) is to work on photos, and these tools may not be used as much for that. But let’s review them anyway for the sake of completeness.

Operating on Raster Graphics

To help understand what the program is doing, remember that raster graphics are based in individual pixels (as opposed to vector graphics, which are described by equations), and each pixel in an image is perfectly described by a series of numbers. A program like GIMP uses clever algorithms to manipulate those numbers. After all, that is about all a computer program can do, when you come down to it.

The Color Tools

Color Picker (O)

This tool lets you click on a pixel anywhere in an image, and copy the color numbers. GIMP operates in RGB space right now (CMYK will, we hope, arrive someday; there is a plugin that partially fixes this, called Separate+. But if you need to do top quality color print production, GIMP is not the best tool right now.) The color picker simply picks up the RGB values on a pixel when you click on it, and if you select “Use info window” at the bottom of the options you can see what the numbers are:

Color Picker Information Window
Color Picker Information Window

You can also use it to set the Foreground Color, the Background Color, and add it to the Palette. For sampling, you can use more than one pixel by selecting Average or Merged. I think of these as Horizontal versus Vertical. Average will select all of the pixels around the one you click on, and the Radius setting determines how many it picks. That is the Horizontal approach. Merged selects the same pixel in all layers of a multi-layer image, and I always think of layers as a vertical stack, so this option goes vertically through the stack. The Color Picker is a tool you use frequently, hence it is in the Toolbox, and has Keyboard shortcut

Color Balance

This lets you adjust the relative values of the RGB numbers to correct an imbalance. For example, I scanned in some old slides (50 years old, about) that had become excessively green. I could moderate that here. Tip: Check the Preview box and you can see your photo change as you move the sliders.

Hue-Saturation

Saturation is essentially the intensity of color in general. If you reduce the saturation to zero in GIMP, you will end up with a gray scale image. Increasing the saturation can improve some pictures, but don’t go too far or it will be garish. Hue refers to the dominant color, and again this could be helpful with my overly green slides. But use this with layers and a selected object and you can change the color of that object. Think of this like those online shopping tools where you can see the same shirt in 8 different colors.

For more on these concepts, see this page at Leigh Cotnoir.

Colorize

This lets you see the image as if there was a colored filter in front of it. This is a way that could take a photo and give it a bit of the old-time sepia-tone look.

Brightness-Contrast

Back in the old days of black-and-white television (kids, ask your grandparents) I heard a comedian tell a joke about how dumb most TV is. He said “I tried increasing the brightness, but it didn’t work.” So what these settings do is increase the overall amount of light (Brightness), and how much contrast there is between light and dark areas. Since this is working on the entire image, it is a bit crude, but can be helpful in seeing what you have to work with if the image is under- or over-exposed. To do a better job, you would want to work with the Levels tool or the Curves Tool

Threshold

This tool turns the image into a purely black-and-white image, and I do mean pure black or pure white. There are no shades of gray here. As you move the slider left towards a setting of zero, more and more pixels turn pure white, and as you move it to the right towards 255, more and more pixels turn pure black. An interesting effect, but by itself probably of limited usefulness. As with all tools, their full usefulness starts to be revealed when we get to Layers.

Levels

This tool lets you adjust Hue and Contrast by channels. If you leave it at Value, it affects all colors equally. But pick a color from the drop-down, and you can increase or decrease the color without affecting the other channels, so this is a bit easier to work with than the Hue-Saturation tool in that respect. Note that if you increase one color, you decrease the complementary color, and vice-versa. For example, adding more Red would also mean reducing the amount of its complementary color Cyan. For more on this, see Complementary Colors.

Curves

This is the most useful tool for adjusting the color and brightness of an image because you can get smooth transitions in the changes you make. When you select this tool, a window pops up with a 45-degree line bisecting a graph. You make changes by clicking and dragging the line to create a smooth curve. This makes your changes less abrupt since nearby values are dragged along in the curve. As before, you have various channels. Value will affect the entire image, Red, Green, and Blue affect those colors, and Alpha affects the transparency (important when we get to layers).

Posterize

This tool reduces the number of colors in your image by making closely similar pixels have the identical color. The fewer the colors, the more blotchy the image, whereas at the highest setting it is pretty close to the original image. The posterize level is basically the exponent in determining the number of colors. Since there are three channels (RGB), a setting of 3 will give you eight colors (2^3=8). The max is 256, which is giving each color the full range of values.

Desaturate

This turns the image into a gray scale or black-and-white image, however the original RGB values are all preserved, which can be handy for later editing.

Summary

This completes our look at the various Color Tools GIMP offers, and there are quite a few of them, to be sure. As with other tools, a little practice will help a lot in getting comfortable with these Color Tools. I recommend opening a photograph in GIMP, and giving each of them a try. That will make things a lot clearer. We still have a few more topics to address in getting the basics going. Next up will be Miscellaneous Tools, then Brushes, and then the really big one, Layers. So there is still lots to go before we get into actual photo work. So please come along on the journey.