Apply 4 color 2d colormap to data

Hi,

I have two 1800x3600 matrices with values between 0-1, and have generated a four-color 2d colormap with the below code, where the x axis would correspond to the values in one matrix and y axis would correspond to the values in the other. I want to use the scheme to colorcode each element with the same index in the two matrices, simlar to the attached figure. Any pointers to accomplish this would be appreciated!

c{1} = [1 0 0]; % specify 4 colors
c{2} = [0 0 0];
c{3} = [1 0 1];
c{4} = [0 0 1];
C = reshape(vertcat(c{:}), 2, 2, []);
n = 700; % number of pixels
img = zeros(n, n, 3);
for i=1:3
    img(:,:,i) = interp2([0 1], [0 1], C(:,:,i), linspace(0,1,n), linspace(0,1,n).');
end
imshow(img);