Ignoring the colour, there are mirrors with/without
a) Auto-dimming or not (you can tell the with and without the lever)
b) Microphones (you can tell this if there is more than one lead coming out)
c) Garage door opener (you can tell this with the 3 buttons at the bottom, but USA version different to rest-of-world - different frequencies)
In other older models there are also versions with memory for the position.
There may also be differences with mirrors for cars with interior lighting pack and not (such as on SLK)
The exterior dimming mirror function is controlled by interior mirror, so if you replace a dimming mirror with one that doesn't, then outside won't dip.
Some cars the mirror plugs directly into OCP, others have different wiring. So putting a dimming mirror on a car that doesn't can be physically easy or need more wiring. Car *should* be coded after adding dimming mirror to a car that doesn't so that it un-dims when you select reverse etc etc
Cheers
Richard