Displaying Mapserver raster in ArcMap gives 256 colours

[POWER Brendan]

I have installed ms4w on a windows box with GDAL, etc...  From here I
created a sample map (mapfile below) sourcing data from ecw images.
When I use the WMS service via an intranet, I get a full colour image
through the GetMap &LAYER command, however, when using ArcMap, I only
get a 256 colour image.  Is there something I am doing wrong?
Any assistance would be appreciated.

NAME TestMap
STATUS ON
SIZE 640 480
EXTENT 116 -24 120 -22
UNITS METERS
SHAPEPATH "d:\Zone_50"
IMAGETYPE FCADIType
OUTPUTFORMAT
  NAME FCADIType
  DRIVER "GDAL/jpeg"
  MIMETYPE "image/jpeg"
  IMAGEMODE RGB
  EXTENSION "jpg"
  FORMATOPTION "INTERLACE=OFF"
  FORMATOPTION "QUALITY=100"
  FORMATOPTION "OUTPUT_TYPE=RASTER"
END
WEB
  IMAGEPATH "/ms4w/tmp/ms_tmp"
  IMAGEURL "/ms_tmp/"
  LOG "/ms4w/tmp/ms_tmp"
  METADATA
    "wms_title" "Metro Photosa"
    "wms_onlineresource"
"http://pc6-089/cgi-bin/mapserv.exe?map=/ms4w/apache/cgi-bin/test.map"
    "wms_srs" "EPSG:4283"
    "wms_feature_info_mime_type" "text/html"
    "wms_abstract" "Some aerials that I could get easily"
  END
END

PROJECTION
  "init=epsg:4283"
END

# MAPSHEETS ---------------------------------------
# Mount Lionel Mapsheet
LAYER
  NAME mount_lionel_100K
  TYPE RASTER
  STATUS ON
  DATA Mt_Lionel_2452_Aug_2004_Mosaic.ecw
  PROJECTION
    "init=epsg:28350"
  END
  METADATA
    "wms_title" "Mount_Lionel_100K"
    "wms_dataurl_format" "image/jpeg"
    "wms_format" "image/jpeg"
    "wms_srs"            "EPSG:4283"
  END
  CLASS
    NAME "mount_lionel_100K"
    # TEMPLATE
  END
END


END # Map File

My apologies for any cross post - last time I tried to send a variation
on this message it bounced...

Brendan Power
Manager GIS
Fire & Emergency Services Authority of WA

address:	480 Hay Street Perth WA 6000
postal:		Po Box P1174 Perth WA 6844

phone:	9323 9351	mobile:	0417 918 809
fax: 	9323 9384	web: 		www.fesa.wa.gov.au