How do I change the colour / labels of individual elements in a layer?

This is controlled in the Mapserver map file layer entry.

Note: See http://mapserver.gis.umn.edu/ for more detail about what can be specified in a map file.

Two simple examples are given below:

Example 1

LAYER

	NAME "GTOPO 30 Antarctica"

	TYPE RASTER

	STATUS ON

	DATA "antarctica/dem/ANTARCPS.DEM"

	CLASS

		NAME "0 - 30"

		EXPRESSION ([pixel] >= 0 AND [pixel] < 30)

		COLOR 255 255 255

	END

	CLASS

		NAME "30 - 60"

		EXPRESSION ([pixel] >= 30 AND [pixel] < 60)

		COLOR 253 253 253

	END

...

...

END

With this entry, all components in the USGS ASCII DEM layer with a value between 0 and 30 would be coloured white, all those with a value of 30 to 60 would be almost white, etc.

Example 2

LAYER

	NAME "Arctic Bathymetric (IBCAO)"

	TYPE RASTER

	STATUS ON

	DATA "arctic/ibcao/IBCAO_arcinfo/ver1_polar/hdr.adf"

	CLASS

		NAME "-5600 - -5550"

		TEXT "-5600 - -5550"

		LABEL

			COLOR 33 33 66

			SIZE medium

			POSITION auto

		END

		EXPRESSION ([pixel] >= -5600 AND [pixel] < -5550)

		COLOR 0 0 0

	END

	CLASS

		NAME "-5550 - -5500"

		TEXT "-5550 - -5500"

		LABEL

			COLOR 33 33 66

			SIZE medium

			POSITION auto

		END

		EXPRESSION ([pixel] >= -5550 AND [pixel] < -5500)

		COLOR 0 0 10

	END

...

...

END

This entry would take the Arc/Info Binary Grid (.adf) vector data and would label and colour regions based on pixel value.