Change random color selection to cycle

[rsignell-usgs]

@hdsingh, the current random color selection currently employed does not work well for small numbers of colors in the color map. Several times I've got the same color twice in a row, which is bad!

Instead, we should cycle through the small number of colors in order.

[hdsingh]

Cycling through a small number of colors in order is good idea and would certainly solve the problem. I would implement this soon. Thanks for this suggestion!