Making small raster areas

[N R A]

Can someone help on this problem. I've got our DTM in (finally - thanks 
to all who helped), and drawn it. The range of values if such that large 
areas look the same colour - I assume this is a limitation of GRASS with 
a pallette of 255 colours ??

If I was to select a smaller area, e.g. a single river catchment, how 
could I change the colour range such that only the values in the smaller 
area were used - thus increasing the spread of colours.

Can I copy a chunk of the large DTM to produce a small raster ??

Thanks for any help,

Steve Culshaw
NRA North West
e-mail : nra at cix.compulink.co.uk (use this for general usage)
or         sculshaw at cix.compulink.co.uk (Private - only checked at best 
biweekly)