You can get the "brightness" of each color channel by (optionally) extracting the color channels:
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
To get the brightness for a particular pixel, for example at row 2, column 42, you can just reference it:
redValue = redChannel(2, 42);
greenValue = greenChannel (2, 42);
blueValue = blueChannel (2, 42);
Or just use the original rgb image:
redValue = rgbImage(2, 42, 1);
greenValue = rgbImage(2, 42, 2);
blueValue = rgbImage(2, 42, 3);
