Hi NewSteve ,
That was quick! Two images already.
Yes you can add the red green and blue channels to make a colour image of the Horsehead nebula in Orion. Don't concern yourself if the colours should look a bit odd at first attempt, maybe they won't, it probably only happens to me. Regarding the Andromeda galaxy, M31, there's only another 100 odd in the Messier catalogue, and I only remember a handful at the best of times! M51 as you'll soon learn is the Whirlpool galaxy. I had a quick look at the software I believe you use to calibrate etc. In the 'Stack Method Tab' tick the 'Convert OSC images to colour' box. Just try each one of the Bayer patterns in turn and apparently it should show you the result. If it mentions each pattern, then you can directly select RGGB.
I hope that helps a bit, and more to the point that it works. Maybe you'd then post the colour image.
Cheers,
Ray