Drawing circles with multi-colored patterns in the annulus

This code gives me a circle with a green center and red annulus. What's the simplest way to create patterns within the annulus, like those shown below in blue and yellow?

xpix = 912;
ypix = 1140;
rad = 200;

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);

% Center
ii = r <= rad/1.5;
RGB(:,:,2)=255*ii;

% Annulus
ii = r <= rad & ~ii;
RGB(:,:,1)=255*ii;

imshow(RGB);
imwrite(RGB,'filter.bmp');

Answer 1

[KSSV]

clc; clear all ;
M = 10000 ;
N = M/4 ;   % Four parts here
R1 = 0.5 ;% Radius of circle1
R2 = 1. ; % Radius of circle 2
th = linspace(0,2*pi,M) ;  % Angle 0 to 360
% polar coordinates
x1 = R1*cos(th) ; y1 = R1*sin(th) ;  
x2 = R2*cos(th) ; y2 = R2*sin(th) ;
%%Arrnage cooridnates into four parts 
X1 = reshape(x1,[N,4])' ;
Y1 = reshape(y1,[N,4])' ;
X2 = fliplr(reshape(x2,[N,4])') ;
Y2 = fliplr(reshape(y2,[N,4])' );
% fill color
figure
hold on
for i = 1:4
    if mod(i,2)
       patch([X1(i,:) X2(i,:)],[Y1(i,:) Y2(i,:)],'r','edgecolor','none')
    else
        patch([X1(i,:) X2(i,:)],[Y1(i,:) Y2(i,:)],'b','edgecolor','none')
    end
end
axis equal

Answer 2

[Thorsten]

You can generate your circles using my function plot annulus

plotannulus(0, 0, 5, 10, [0 0 1; 1 1 0], deg2rad(90))
plotannulus(0, 0, 5, 10, repmat([0 0 1; 1 1 0], 2, 1), deg2rad(45))
plotannulus(0, 0, 5, 10, repmat([0 0 1; 1 1 0], 3, 1), deg2rad(60))
plotannulus(0, 0, 5, 10, repmat([0 0 1; 1 1 0], 4, 1), deg2rad(0))

The plotannulus function:

function h = plotannulus(cx, cy, r1, r2, col, theta0)
%PLOTANNULUS
%
%   H = PLOTANNULUS(CX, CY, R1, R2, COL, [THETA0])
%
%   THETA0 Start angle in radians of the first color, if more than one color
%       for the annulus is given. Default is 0.
%
%  Examples
%     plotannulus(0, 0, 5, 10, repmat([0 0 1; 1 1 0], 2, 1), deg2rad(45))
%     plotannulus(0, 0, 5, 10, parula(12))
%     plotannulus(0, 0, 5, 10, jet)

if nargin < 5, col = [0.5 0.5 0.5]; end
if nargin < 6, theta0 = 0; end

Ncol = size(col, 1);

delta_theta = 2*pi/Ncol;
Theta = linspace(0, (2*pi - delta_theta), Ncol) + theta0;
Npoints = 360/Ncol; % number of points along the radii to approximate the
                    % circle
for i = 1:Ncol
  theta = linspace(Theta(i), Theta(i)+delta_theta, Npoints);
  x1 = r1*cos(theta);
  y1 = r1*sin(theta);

    x2 = r2*cos(theta);
    y2 = r2*sin(theta);
    h(i) = patch([x1 fliplr(x2)] + cx,...
                 [y1 fliplr(y2)] + cy,...
                 col(i,:), 'EdgeColor', col(i,:));
  end

if nargout == 0
  clear h
end