Do you have the ETM? If not,
download it from here, sections 6500 have the wiring colors. I think your car will be loaded up with the upgraded BMW Premium system, so your wiring will be a bit different than most cars here... I'm lucky my car even had a stereo, LOL.
IIRC you'll have only five speaker wires - the four normal channels with a single common ground to the crossover/amp in the trunk. Then you'll have the usual power, acc, ground, amp power, brightness and power antenna wires.
Be aware that you'll likely have to wire the aftermarket stereo's "memory/battery/acc." wire directly off the battery or some other good unswitched power source. I pulled mine off the hot lead (thick red wire?) in the hazard light switch. Newer head units tend to run off the battery wire, not the switched wire like the OEM BMW head unit did.