I study and have an interest in genetics, I know that coloured eyes originated in cold temperatures, and darker features were from the hot temperatures, but is dark (brown) hair actually a light type of hair (since it is brown) and how does one have dark hair with blue/green eyes?

Did you mean to say light colored eyes in cold?
You get variations because eye and hair color are two different genes, and although they often go together, they don't have to, especially with light skinned people. I don't know if dark brown hair is considered 'light' genetically or not, but I suspect that it isn't really part of the technical lexicon. Hair color can change with age, time in the sun, and even diet. And two dark eyed parents can have a bluegreen eyed child. Each parent has the recessive gene for blue.