You can use a single-value visualization, where you can set a rangemap for color-coding. See Single value visualization in the Dashboards and Visualizations manual. You can also use a radial gauge, as aweitzman suggested, or another type of gauge (see the link aweitzman provided, it is in the same topic).
I think you'd want something like a gauge:
http://docs.splunk.com/Documentation/Splunk/6.1.3/Viz/Visualizationreference#About_gauges