All depends on what you want to end up doing with your study. Is this more of a personal interest thing that you would eventually share on a site like this? Or do you want to eventually get it published within a scientific journal?
If it's the former, simply photographing the animals you're interested in and making observations of them in the field, and then looking at comparisons between the different populations would be a good start. Then you can have hypothesise what is occurring and then trigger a discussion around it. We'll never be able to actually figure out what is the driver by the different colour forms, which could just potentially be random chance, but it would be interesting.
If it's the later, things become a lot more complex. I would highly recommend talking to a few different ecologists about what you're aiming for and trying to do. They will then be able to give you advice and potential assistance with designing and potentially carrying out the project. The hardest part of a project like this is creating a sound methodology to test for drivers behind the different colour phases. This is even harder without experiments, so it is important to remember that what you do will likely only be able to describe and speculate about what are the drivers behind these differences.
Obtaining an animal ethics permit for your proposed research will also be another step you'll likely need. Even though you're mainly photographing them, because you're doing research on a vertebrate you'll need an approved ethics permit from a university or similar institution. Might sound silly, but during one of my undergraduate courses an ethics permit was required for people doing a short behavioural projects on bats and kangaroos even though they were just photographing them. Most journals these days will also not accept manuscripts if the research was done without an ethics permit.
Hopefully all of that doesn't put you off your idea though. It is a lot of work and planning to do ecological studies (my entire year will be devoted to reading, planning and applying for permits as a part of my PhD), but they're worth it in the end!
If you have any other questions feel free to ask.
Cheers, Cameron