[ferret_users] About color palette

[Weipeng Zheng <zhengwp@xxxxxxxxxxxxxx>]

Dear ferreters,

I want to plot a figure with the value range from -10 to +10 and I 
prefer to use the grayscale palette and have the white in the center:

like -10 to 0 ( from darkgray to white)
    0 to +10 ( from white to darkgray)

I don't know how to define this palette, can somebody give me this 
palette or an example.
I fill confusing about the percentage and the RGB code.

Thanks in advance.

Regards!

Weipeng