DOCSP-3439: [Charts] Color palette customization (#84)

* DOCSP-3439: [Charts] Color palette customization

Author: steveren

source/customize-charts/general-customization.txt

@@ -100,3 +100,58 @@ To set this option:
 #. In the :guilabel:`General Customization` section, set the
    :guilabel:`Data Category Labels` toggle switch to the desired
    setting.
+
+Color Palette Customization
+---------------------------
+
+Palette customization allows you to select custom colors for your
+chart. To enable color customization, select the
+:guilabel:`Customization` tab of the :guilabel:`Chart Type` column.
+A toggle switch labeled :guilabel:`Custom Color Palette` appears in
+the :guilabel:`General Customization` section. 
+
+Most chart types allow you to customize individual colors in their
+palette. The :ref:`heatmap <heatmap-chart-ref>` and :ref:`colored bar
+and column <column-bar-chart-ref>` types use a gradient color scale,
+while other types use a custom color picker. The gradient color
+scale allows you to choose from a set of pre-defined gradients.
+
+.. figure:: /images/charts/custom-gradient.png
+   :figwidth: 251px
+   :alt: Custom color palette selection
+
+Other chart types use a color picker for selecting a customized
+set of colors.
+
+.. figure:: /images/charts/custom-color-palette.png
+   :figwidth: 251px
+   :alt: Custom color palette selection
+
+For chart types which use a color picker for customization,
+each numbered color swatch corresponds to a data series on the chart.
+Select a color swatch to modify the color of its corresponding
+data series. You can select a new color with a color slider, by
+entering a hex value, by entering RGB values, or by choosing a
+predefined color. You can also drag and drop the existing color
+selections if you want to use the current color choices but change
+which color goes with which data series.
+
+.. note::
+
+   By default, the custom color picker shows color swatches for 10
+   data series, but you can only modify as many of the swatches as
+   you have data series. The palette can expand to show 20 data series. 
+   Charts will use as many colors as needed to show your chart,
+   starting from the first color. For example, if you have only two
+   data series in your chart, only the colors numbered ``1`` and
+   ``2`` have any effect on your chart.
+
+To close the color customization tool, click somewhere outside it.
+To revert the entire chart to its default colors, set the
+:guilabel:`Custom Color Palette` toggle switch back to :guilabel:`Off`.
+
+Applicable Chart Types
+~~~~~~~~~~~~~~~~~~~~~~
+
+This customization option is available for all chart types except
+:ref:`Text <chart-type-text-charts>`.