How to change the intensity of a circle/ring in an image?

m = 256 ;
n = 256 ; 
I = 100*ones(m,n) ; 
% center of image 
[X,Y] = meshgrid(1:n,1:m) ; 
C = [mean(1:n) ,mean(1:m)] ; 
th = linspace(0,2*pi,m) ; 
% First circle 
R1 = 32 ; 
x1 = C(1)+R1*cos(th) ; 
y1 = C(2)+R1*sin(th) ; 
idx1 = inpolygon(X,Y,x1,y1) ; 
I(idx1) = 250 ; 
% SEcond circle 
R2 = 64 ; 
x2 = C(1)+R2*cos(th) ; 
y2 = C(2)+R2*sin(th) ; 
idx2 = inpolygon(X,Y,x2,y2) ; 
I(logical(idx2-idx1)) = 200 ; 
% Third circle 
R3 = 96 ; 
x2 = C(1)+R3*cos(th) ; 
y2 = C(2)+R3*sin(th) ; 
idx3 = inpolygon(X,Y,x2,y2) ; 
I(logical(idx3-idx2)) = 150 ; 
% plot
imagesc(I)