By default, the contrast is set to 1 and the brightness to 0.5. This fits the colormap exactly within the color bar plotted under the mapplot image. The center of the colormap can be moved up or down the color bar by changing the colormap brightness. This is best done interactively in mapplot, but can also be set on the command line with the mapcolor command. The width of the colormap compared to that of the color bar can also be changed by altering the contrast.
High contrast equates to a colormap that occupies only a small fraction of the color bar.
Low contrast equates to a colormap extent that exceeds the width of the color bar, such that only a small range of its colors are visible. Negative contrasts act the same as positive contrasts except that the colormap is reversed. Again, the contrast is best set interactively in mapplot, but can be preset with the 'mapcolor' command. Note that to completely flip the sense of a color map you should first run the mapcolor command without arguments, to see what the current brightness and contrast are. To see what the current colormap settings are, invoke the colormap command with no arguments.