Matrix of fading colors

I'm trying to make a 256x256 matrix of colors fading into each other. The top left corner is red, the top right is purple, the bottom right is blue, and the bottom left is green. The blue and the red fade into each other, which is easy to figure out. The problem is the green fades out radially, in both directions, and I can't get my image to do that. My code so far is:

mat3d(:,:,1) = zeros(256,256); 
mat3d(:,:,2) = zeros(256,256);
mat3d(:,:,3) = zeros(256,256);

increase = linspace(0,1,256);
decrease = linspace(1,0,256);

for i = 1:256
     mat3d(i,:,1) = decrease(i);
     mat3d(:,i,3) = increase(i);

       for j = 1:256
      dist = sqrt((256-i)^2+(256-j)^2);

       end
end

I can't figure out how to write the last for loop.

Answer 1

[DGM]

Loops aren't needed.

% don't embed parameters into the code
outsize = [256 256]; % [y x]

% i'm assuming we live prior to R2016b
x = linspace(0,1,outsize(2));
y = linspace(1,0,outsize(1)); % because the image origin is at the top
[X Y] = meshgrid(x,y);
R = 1 - sqrt(X.^2 + Y.^2)/sqrt(2);

% assemble the image
outpict = cat(3,Y,R,X);

% show it
imshow(outpict)