How to draw a circle with a red annulus and green center?

I'm trying to draw a circle with a red annulus and green center using RGB in MATLAB R2014b. However, since I'm currently drawing the red circle first, then layering the smaller green circle over it, the green turns to yellow. How do I either prevent the colors from adding (so the green shows as green) or draw the red annulus only (instead of the full circle)?

xpix = 1140;
ypix = 912;
rad = 150;
RGB(1:xpix,1:ypix,1:3)=0;
[x y] = find(RGB==0);
xc = ceil(xpix/2);
yc = ceil(ypix/2);

for i=1:20
    radius = (rad-20*i).^2;
    r = find(((x-xc).^2+(y-yc).^2) <=radius);

      for j=1:size(r,1)
          if(mod(i,2)==0)
              RGB(x(r(j)),y(r(j)),1)=255; % 1=red
          elseif (mod(i,10)==1)
              RGB(x(r(j)),y(r(j)),2)=255; % 2=green
          end
      end
  end

imshow(RGB);
imwrite(RGB,'filter_1.jpg','BitDepth',8);

This is what I currently have:

[Image Analyst]

Explain what you're doing. What is the point with the mod() calls? What's with the for loops?

Answer 1

[Robert]

How about a simpler implementation without the loops:

xpix = 1140;
ypix = 912;
rad = 150;

% Use zeros to initialize RGB. No risk of modifying an existing RGB. Use
% ypix for the rows and xpix for the columns to make x the horizontal size
% and y the vertical size.
RGB = zeros(ypix,xpix,3,'uint8');
[x,y] = meshgrid(1:xpix,1:ypix);
xc = (xpix+1)/2;
yc = (ypix+1)/2;
r = sqrt((x-xc).^2+(y-yc).^2);

ii = r <= rad/2; % Inside half our radius
RGB(:,:,2)=255*ii; % Green

ii = r <= rad & ~ii; % Inside our radius but not already made green
RGB(:,:,1)=255*ii; % Red

imshow(RGB);

[M]

Perfect - thanks so much Robert!