An example of indicator, that draws bars for the specified instrument in a separate window.

The color of bars changes randomly every N ticks. The N parameter of indicator is made as external so that you can adjust it manually (the Inputs tab of indicator properties dialog box). Note, that initially there are 8 colors set for the plot1 graphical plotting (with the DRAW_COLOR_BARS style) using the #property compiler directive. Then, in the OnCalculate() function the color is chosen at random from 14 colors stored in the colors[] array.

See also The Drawing Styles in MQL5 article.