** This bug is fixed at ArcGIS Desktop 9.1 **
The ArcGIS Spatial Analyst Histogram tool, on the Spatial Analyst toolbar, fails if the raster is rendered using a colormap.
Raster layers symbolized with a colormap renderer are not supported by the Histogram tool.
Change the renderer to Unique Values.
- Right-click the raster layer in the table of contents and click Properties.
- Click the Symbology tab and click Unique Values from the list of renderers.
- Click OK, then click the Histogram tool, making sure that the raster layer is first selected in the Layer drop-down list on the Spatial Analyst toolbar.