The Dot-scope is supposed to help you acquiring a target at close range faster and this it does, it allows you to aim with both eyes open and keep up
your awareness of the surroundings. See
here
(Aimpoint),
here (Reflex sight) and
here
(holographic sight) for further information.
If you want a scope with magnification, your best (and by far most expensive) bet would be a
trijicon ACOG. They are available with different magnification and different
reticles, but they're more expensive than many good rifles.
The different colors can help you see the reticle against different backgrounds by having a clear contrast. In the woods for example, a red reticle is
preferable to a green one due to visibility. At night, a green reticle might be easier on the eye than a red one.
If your primary need is magnification, I'd go with a good, old-fashioned scope with selectable magnification, like
this one, at a reasonable price.
I hope, this was helpful
Edit to add:
If you want a sope with illuminated reticles for night time shooting,
this
scope here seems to be quite good at a nice price.
[edit on 21/10/2009 by Swordbeast]
But I only used it when in brush situations or when on drives for white tail. Did
not even know about the others. 
