how to plot pcolor with contour ??

sir i am trying to plot pcolor with contour using the following code

x=61:1:100;
y=1:1:40;
figure;
pcolor (x,y,m1); % m1 is a gridded data file of trend (increasing or decreasing) having dimension 40*40
hold on;
plot (a,b); %a,b are the co ordinates of the indian map
hold on
contour (x,y,c1); %c1 is the data file of correlation coefficient having dimenstion 40*40
hold off

but when I plot the contour plot, it turns the whole pcolor plot a nasty shade of green or something. please help