Want different color over different regions in a map

Hi,

I am trying to draw a map for a region and divide it in four parts and
color each parts separately. I am attaching the plot showing the region
I want in which suppose I want to divide it in four parts by drawing a
line along 50N latitude and 100W longitude. I want to color only the
land part.
Can anybody please give me some suggestions about how to do it.

Thanks,
Soumik

-- 
"Numbing the pain for a while will make it worse when you finally feel 
it.
  ~J.K. Rowling"
*****************************************
Soumik Basu
Graduate Student, Research Assistant
International Arctic Research Center
University of Alaska Fairbanks
Fairbanks,Alaska,USA
*****************************************

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