As indicated by the topic name, I need some help creating a mask.
As you can see in the background of this image, there are what is intended to be siren lights.
There weren't any siren lights on set so it was done by having someone switch two lights with a red and a blue gel on and off. Unfortunately, I only have a small period of the shot where the switching between the lights is consistent enough and not erratic. So to keep the shot on for as long as we want it, what I think I need to do is mask that part of it and layer it on top of the rest of the shot.
If someone can direct me on how to do that or possibly recommend a better solution, that would be great.
You can make a mask with the Animatte Effect. Search for that in the Help Menu for details on how to use it. It's pretty straightforward.
If I am understanding what you are trying to achieve, this may only work effectively if your shot is locked down and the camera is not moving. If you need anything more than very basic tracking, you are probably going to need to out of Avid to do this convincingly.
depending on what you have at your disposal, if the camera is moving, the typical technique is to stabilize the shot, mask it off, and then un-stabilize the shot by adding a reverse of the original stabilization.
Boris can do the stabilize, so i'm assuming it can do the reverse as well.
Mocha, fusion, after effects could all also help if you feel the need to leave Media Composer.
If the shot is locked off, then yes, the animatte will work quite well...feathering the edges where the existing lights intersect the body may be the most difficult area.
Adding the lights in with an additive or screen technique will help to hide these areas.
Again, using something like fusion to create your lights would be a great way to proceed, as you could actually have the lights spin through the shot, as if they were rotating from the top of the police car.