Colour Scaling Control

Hi. 

I try to adjust the Scalar mapping for the RGB or IsoContour and having some
problem in that. As far as I know, the VisAD is using the linear scaling
method to map the colour for the values. My requirement is for a range of
figures with negative and positive values, I want to specify a certain
colour for -ve values, another colour for zero, and different colour again
for +ve values. On top of that, the linear scaling can be applied for
individual regions to project a better view. May I know how should I do
that? Thank you for all who read my emails and response to my mail. Have a
nice day.