This is because a white background was desired and the foreground color is used to write text on top of colors that are different from the background color. The six color indices from 2 through 7 are used. These indices are associated with the primary and secondary colors in the color table. Line 14 calls GCA to produce the cell array. Cell arrays provide for a way of displaying raster image data along with text, lines, markers, and filled areas.
Some data are more suitable for display as a raster image.
For example, you may have data from scientific equipment that is in the form of an array of color intensities. Or you may have a screen dump from a graphics output device that is in the form of a raster image. These arrays would have to be converted to a form suitable for using GCA, but once that is done you can integrate such data with the rest of NCAR graphics and display them on multiple output devices. One disadvantage in using GCA is the case where the resolution of the cell array is not well matched with the resolution of the graphics output device.