A color map is a set of values that are associated with colors. Color maps are used to display a single-band raster consistently with the same colors. Each pixel value is associated with a color, defined as a set of red, green, and blue (RGB) values. Since each value has a distinct color associated with it, it will always display the same way each time you open it in a program that can read a raster with a color map. 
Color maps are capable of supporting any bit depth, except FLOAT. They can also support positive and negative values, and the color maps can contain missing color mapped values. When displaying a dataset with a color colormap containing missing value, the pixels with that value will not be displayed.
By default, a raster dataset with a color map will always be displayed using the associated color map. If you want to change the raster dataset's appearance, change the colors or the color map file in the Colormap renderer panel (accessible in the raster dataset's Layer Properties dialog box on the Symbology tab) or use the Unique Values renderer. See Renderers used to display raster data to learn more about the different types of renderers.
The easiest way to see if your raster dataset has a color map associated with it is to open the Raster Dataset Properties (in ArcCatalog) or the Layer Properties (in ArcMap); on the Source tab the colormap property will be present if there is a color map associated with the raster dataset.
Color maps are associated with the raster dataset in different ways, depending on the raster format. Some raster formats, such as .bil, .bip, or .bsq, have the color map information stored in an auxiliary file next to the raster. Other formats, such as .img, .tif, or ArcSDE, have the color map information stored internally within the raster dataset itself.
Using geoprocessing tools, you can copy or delete a color map for a raster dataset. The Add Colormap tool allows you to copy the color map schema from an existing color-mapped raster dataset, .clr file, or .act file to another raster dataset. The Delete Colormap tool allows you to delete the color map from an existing raster dataset. Not all raster formats are applicable for these two geoprocessing tools; please refer to Supported raster dataset file formats to see which formats can support color maps.
Some applications write internal color maps in a way ArcGIS cannot alter. This may happen with some IMG or TIFF files. Therefore, the Delete Colormap tool will not be able to remove their color maps.
Raster attribute tables that contain Red, Green, and Blue fields are not valid color maps and cannot be deleted using the Delete Colormap tool.