Help for change the color precision in imageplot with a colorbar

Hi, everyone,

        I met a problem in changing the color precision in imageplot 
with a colorbar corresponded.  As we know,  a color represents a range 
of values in the imageplot so I want to reduce the number of the 
colors.  The definition of this conception is in the ColorMapper which 
has an attribute, steps(value default is 256).  To change the color 
precision of a colorbar,  I can motiify "steps" to reduce the number of 
colors present in the colorbar, but the imageplot does not change when 
the colorbar color precision changed.  How to change the number of 
colors  representing in the imageplot?

Thanks  a lot for any help,

Lin