If your raster has a color map, it is displayed with the Colormap rendered by default. You can change this to use the RGB renderer.
- Click the Add Data button to add a raster to ArcMap.
- Click Windows on the Main menu to view the Image Analysis window, and click Image Analysis.
- Click the raster layer in the Image Analysis window.
- Click the Colormap to RGB button in the Processing section of the window.
This adds a temporary raster dataset layer to the table of contents.