Sometimes you may want to change the RGB value of a color in the color map, instead of changing which color is assigned to a particular name. For example, you may need to make a black and white picture and need to emphasize the contrast between a red oxygen and a yellow sulphur. This is done with the Edit Colors part of the form. First, choose the color you want changed in the browser. Then move the red, green, and blue sliders until you get the desired color. There are a few additional buttons to help you do this. The white button makes the color white (red = blue = green = 1.0) and the black color makes it black (red = blue = green = 0.0). The default button restores the color to its original RGB values. The tie button is used to make grey colors; when the button is depressed, as you move one slider the rest will follow. Press the tie button again to untie the sliders.
For example, suppose, we don't really like the appearance of the green color and we want to make it darker. To see what is happening we'll change the color of the background, so choose Display in the category field of the upper part of the Color form, then choose Background, and, finally, choose green. The background should become green. The (default) RGB values are 0.20, 0.70, 0.20. To make the color darker, let's bring the RGB values down by moving the red, green and blue sliders to the left. You can see the color changing as you move the sliders, so this way you can easily pick the color you prefer to be named 'green'. The definition can be immediately brought back to the default values by pressing the 'Default' button on the form. Note, that moving the sliders changes both solid and transparent ('green' and 'trans_green') colors (see Chapter § for details).