## Controlling Colors in GrADS

The GrADS Default Colors

GrADS is built with 16 default colors that are used in a variety of applications. Every color in GrADS has a unique color number that is used as an index to identify it in GrADS commands. Complete specifications of the default colors numbered 0 to 15 are given below:

    Col#  Description   Sample    R   G   B

      0   background       0   0   0  (black by default)

      1   foreground     255 255 255  (white by default)

      2   red            250  60  60

      3   green            0 220   0

      4   dark blue       30  60 255

      5   light blue       0 200 200

      6   magenta        240   0 130

      7   yellow         230 220  50

      8   orange         240 130  40

      9   purple         160   0 200

     10   yellow/green   160 230  50

     11   medium blue      0 160 255

     12   dark yellow    230 175  45

     13   aqua             0 210 140

     14   dark purple    130   0 220

     15   gray           170 170 170

Disclaimer: The color samples may not be displayed properly.

The GrADS Default Rainbow Sequence

GrADS creates a default rainbow palette using the following sequence of 13 built-in colors:

9 | 14 | 4 | 11 | 5 | 13 | 3 | 10 | 7 | 12 | 8 | 2 | 6

When drawing contour plots, the default behaviour of GrADS is to color code the contours and select an appropriate contour interval so that each contour is a different color and the colors span the range of the default rainbow sequence. The same principle is behind the selection of default contour intervals for filled contours and shaded grid plots.

The scripts `"cbar.gs"` and `"cbarn.gs"` will draw a color key alongside a plot of filled contours or shaded grid cells; the script uses the `query shades` command to get information about the contour levels and their color shades.

Defining new colors

For some types of displays, the 16 GrADS default colors may not be suitable or adequate. It is possible for the user to define new colors using the `set rgb` command:

`set rgb color# R G B`

For example, let's create a palette of colors for plotting anomalies. We need to define new colors that will be shades of blue and red that range in intensity from fully saturated to very light. White will be the color in the center of the new anomaly palette.

    * These are the BLUE shades
    set rgb 16   0   0 255
    set rgb 17  55  55 255
    set rgb 18 110 110 255
    set rgb 19 165 165 255
    set rgb 20 220 220 255
    * These are the RED shades
    set rgb 21 255 220 220
    set rgb 22 255 165 165
    set rgb 23 255 110 110
    set rgb 24 255  55  55
    set rgb 25 255   0   0

Overriding the Defaults

Now that we have a set of newly defined colors (numbered 16-25), we can override the defaults and specify our anomaly palette with exact contour levels and the colors that go with them. This is accomplished by using the following commands:

`set clevs lev1 lev2 lev3 ... levN
set ccols col1 col2 col3 ... colN
`

Contour levels and the colors that go with them are reset with every execution of `clear` or `display`. Thus, it may be easier to use these commands in a script to avoid typing them over and over again.

Filled Contours or Shaded Grids: If you are specifying the levels and colors for filled contours (`set gxout shaded`) or shaded grid cells (`set gxout grfill`), then the number of colors specified with set ccols must be one larger than the number of contour levels specified with set clevs. Continuing with our example of creating an anomaly palette, the commands would have the following syntax:

`set gxout shaded
set clevs -5 -4 -3 -2 -1 1 2 3 4 5
set ccols 16 17 18 19 20 1 21 22 23 24 25
`

Note the "0" contour level has been omitted, but color number "1" is still in the palette. Drawing a plot with these specified `clevs` and `ccols` and then running the "cbarn.gs" script will result in the following color key:

Here is example using 6 colors and 5 contour levels that shows how the filled contours (or shaded grids) relate to the data values:

    col1:         values <= lev1
    col2:  lev1 < values <= lev2
    col3:  lev2 < values <= lev3
    col4:  lev3 < values <= lev4
    col5:  lev4 < values <= lev5
    col6:  lev5 < values

Line Contours: If you are specifying the levels and colors for line contours (`set gxout contour`), then the number of arguments to set clevs and set ccols should be equal -- one color for each contour.

Plotting Contours of Constant Color

It is sometimes preferable to plot line contours without the rainbow coloring. An example might be a plot with sea level pressure contours in one color (red) and 500 mb height contours overlaid in another color (blue). For drawing all the contours in the same color, use the `set ccolor` command:

`set gxout contour
set ccolor 2
d slp
set ccolor 4
d z(lev=500)
`

Omitting Colors

The default behavior of GrADS when plotting filled contours or shaded grid cells is to colorize all areas. To omit a particular color (or contour level) from the plot, simply assign the background color. For example:

`set gxout shaded
set clevs -5 -4 -3 -2 -1 1 2 3 4 5
set ccols 0 17 18 19 20 0 21 22 23 24 0
`

This example is similar to the one given above, but notice where some of the `ccols` have been set to "0" (the background color). The first, last, and middle colors have been omitted. These commands set up a plot that will only shade areas where the anomalies are between 1 and 5 and -1 and -5. The remaining areas will be black.

Plotting Non-Continuous Index Grids

Plotting grids with index values or non-continuous data (e.g. surface type classification) is simplified by using the graphics output type `fgrid` and the `set fgvals` command.

`set gxout fgrid
set fgvals 1 15 2 5 3 1
d sfctype
`

In this example, the variable `"sfctype"` has three values: `1` represents land, `2` represents oceans, and `3` represents sea ice. These commands would draw a plot with land grid cells filled with color number 15 (gray), ocean grid cells filled with color number 5 (light blue), and sea ice grid cells filled with color number 1 (white). If the first two arguments to set fgvals were omitted, then the land grid cells would not be omitted and only ocean and sea ice grid cells would be colored.