The rose-ringed parakeet (also known as the ring-neck parakeet) was spotted on my first afternoon in London, as I was walking through Kensington Park on my way to the Natural History Museum. While it is native to Africa and the Indian subcontinent, it has been able to adapt and colonize numerous cities within Europe (including London), and can even be spotted within the US.
Wild rose-ringed parakeets have green feathers, and the adult males can be distinguished from the females and immature males by their red-and-black neck ring. The females and immature males either lack the ring or have a pale-to-dark gray neck ring.

The neck rings for birds that have been bred in captivity can range in color to include turquoise, cinnamon, olive, white, blue, viole, gray, and yellow.
The parakeet is native to parts of Africa and India, but has also managed to adapt and colonize numerous cities within EUrope (where they’d either been released at some point or escaped captivity). There are even a few small groups within the United States as well (in Florida, California, and Hawaii).

In terms of where to spot them–I managed to spot this female (or immature male) sitting in a tree in the park, so one can look there. Though they may also show up at bird feeders within cities to eat as well (so you might be able to spot one at a feeder).
For ‘wild’ parakeets, their diet includes buds, fruits, vegetables, nuts, berries, seeds, and cereal grains (depending on location). The diet of ‘feral’ parakeets is somewhat similar, but they will also visit bird feeders as well.
A photography goal is to get a picture of a male ring-neck parakeet, in addition to getting a picture of them within their ‘natural’ range of Africa and/or India. In addition to possibly taking another trip to the Hawaiian islands and seeing if I can spot one of them there as well.
Reference: https://en.wikipedia.org/wiki/rose-ringed_parakeet