Before you start thinking about colour, cost, and shape, it is vital to consider other factors to determine the best sunglasses you can get.
The first and most important consideration is to ensure you get UV protection, which is not something that depends on the darkness of lenses, colour, and price tag.
You should know that plastic and glass lenses would absorb UV light, which means that you should find the ones that feature specific chemicals or lens coating to protect your eyes.