Try this:
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Write out to separate sheets in an Excel workbook.
xlswrite(filename, redChannel, 'Red', 'A1');
xlswrite(filename, greenChannel, 'Green', 'A1');
xlswrite(filename, blueChannel, 'Blue', 'A1');
