You grayscale image (say iRed) is a m*n*1 matrix. (m, n are x and y dimensions). To make the grayscale image iRed to redscale image, do the following computations:
>> iRedMonochrome=cat(3,iRed,zeros(size(iRed)),zeros(size(iRed))); % for red filter [R 0 0]
>> iGreenMonochrome=cat(3,zeros(size(iGreen)),iGreen,zeros(size(iGreen))); % for green filter [0 G 0]
>> iBlueMonochrome=cat(3,zeros(size(iBlue)),zeros(size(iBlue)),iBlue); % for blue filter [0 0 B]
You can then simply add the images:
>> iRGB=iRedMonochrome+iGreenMonochrome+iBlueMonochrome;
