Appearence of a digital elevation model

Here is an elementary question about the way to represent a 
temperature field. I wish to obtain  a 3d picture of a 3d field 
((lat, long, alt)-> temp) on a 3d grid,  with background a digital 
elevation model ((lat,long)->elev), on a 2d grid.  I have  almost got 
it.  I still have two problems. First,  sometimes the DEM appears as 
a DEM in grey in a realistic way, sometimes it appears as a 
polyhderic surface and I do'nt know how to  control  its appearence. 
Second I wish to represent the temperature field by colouring the 
points of a 3d grid; is there a simple way to colour just the points 
with temperature in a given range ?

Thanks

Christian Mullon