For all three colors, you are not getting RGB, but the complementary colors cyan,magenta,yellow.
Here's RGB corrected, but with a black background:
x=-1:.01:1;
y=-1:.01:1;
[X,Y]=meshgrid(x,y);
Z=sqrt(X.^2+Y.^2);
Z(Z>1)=1;
R=zeros(330,330);
G=zeros(330,330);
B=zeros(330,330);
R(1:201,1:201) = 1-Z;
G(76:276,76:276) = 1-Z;
B(1:201,126:326) = 1-Z;
figure
img=cat(3,R,G,B);
h=image(img)