Next: I.2 HSV color system Up: I. Color Space: The Previous: I. Color Space: The Contents Index

# I.1 RGB color system

Remember your (parents') first color television set? Likely it had three little bright colored squares on it: red, green, and blue. And that is exactly what each color on the tube is made of: varying levels of red, green and blue light. Switch all of them off, , then you have black. All of them at maximum, , creates white. Your computer screen works the same way.

A mix of levels of red, green, and blue creates basically any color imaginable. In GMT each color can be represented by the triplet //. For example, 127/255/0 (half red, full green, and no blue) creates a color called chartreuse. The color sliders in the graphics program GIMP are an excellent way to experiment with colors, since they show you in advance how moving one of the color sliders will change the color. As Figure I.1 a shows: increase the red and you will get a more yellow color, while lowering the blue level will turn it into brown.

Figure: Chartreuse in GIMP. (a) Sliders indicate how the color is altered when changing the H, S, V, R, G, or B levels. (b) For a constant hue (here 90°) value increases to the right and saturation increases up, so the ``pure'' color is on the top right.
a b

Is chocolate your favorite color, but you do not know the RGB equivalent values? Then look them up in Figure I.2 or type man gmtcolors for a full list. It's 210/105/30. But GMT makes it easy on you: you can specify pen, fill, and palette colors by any of the more than 500 unique colors found in that file.

Are you very web-savvy and work best with hexadecimal color codes as they are used in HTML? Even that is allowed in GMT. Just start with a hash mark (`#`) and follow with the 2 hexadecimal characters for red, green, and blue. For example, you can use `#79ff00` for chartreuse, `#D2691E` for chocolate.

Figure I.2: The 555 unique color names that can be used in GMT. Lower, upper, or mixed case, as well as the british spelling of ``grey'' are allowed. A4, Letter, and Tabloid sized versions of this RGB chart can be found in the GMT documentation directory.

Next: I.2 HSV color system Up: I. Color Space: The Previous: I. Color Space: The Contents Index