You can still turn left when the light is green, but only when the way is clear of traffic and pedestrians. If the light turns red when you are in the intersection, complete your turn when it is safe.
Very true. It all depends on how did you describe the incident. A factual statement would be:
"I was driving East/West/North/South on XXX street. At the intersection with YYY street and ZZZ (from on-coming direction) did a left-turn in front of my car. The result was a two-vehicle collsion."
After that, the other party would say (s)he waited for the RED light to do the left turn. And you can simplyt ask for proof/witness.
Without any proof that you entered the intersection after the light turned red, this case can only be judged based on the obvious fact quoted above.