Just a comparison of spectra of the brightest O & M spectral class stars in the night sky, Alnitak (O) and Betelgeuse (M). These are at opposite ends of the classes, without going beyond M to brown dwarfs etc. The 'redness' and 'blueness' are very marked. Alnitak has a pretty smooth continuum with a couple of small hydrogen absorption lines. Betelgeuse has abundant and deep absorption lines.
Both are supergiants but it might be interesting to repeat the exercise with Main Sequence stars. AX Mic is the brightest Main Sequence M-class star but is not well-positioned at the moment from here. S Mon could be good for O-class.
One thing that shows up glaringly is the blue limit of the camera sensor of the 650D - the blue end of the Alnitak spectrum is cut off very sharply!
Cheers -
