+ * A class that represents a colour scheme for a feature type. Options supported
+ * are currently
+ * <ul>
+ * <li>a simple colour e.g. Red</li>
+ * <li>colour by label - a colour is generated from the feature description</li>
+ * <li>graduated colour by feature score</li>
+ * <ul>
+ * <li>minimum and maximum score range must be provided</li>
+ * <li>minimum and maximum value colours should be specified</li>
+ * <li>a colour for 'no value' may optionally be provided</li>
+ * <li>colours for intermediate scores are interpolated RGB values</li>
+ * <li>there is an optional threshold above/below which to colour values</li>
+ * <li>the range may be the full value range, or may be limited by the threshold
+ * value</li>
+ * </ul>
+ * <li>colour by (text) value of a named attribute</li> <li>graduated colour by
+ * (numeric) value of a named attribute</li> </ul>