The 'custom palette' option does not currently exist. If you wish to change the colors of an element within a chart, simply edit the properties of said element and change the color in the Appearance tab, Series Style, Border and Line tab, Color: field.
Example: I have a line chart whose data element, by default, appears as green on the chart. The users want "Steel Blue" instead. To change this, I edit the chart properties, edit the Values element, swithc to Appearance tab, select the Series Style button, and change the Color: value to steel blue (Border and LIne tab in Series Style... it is the first tab).