I was playing Pokemon Go and the map around me has a blue and green color scheme. Does this mean anything or is it just for cosmetic purposes?
The different colours you see are different "environments". To quote the Pokemon GO FAQ:
Some wild Pokémon appear only in certain environments and climates. For example, some Pokémon may appear only near lakes, oceans, or other bodies of water
"Environments" are based off GPS maps, and are somewhat accurate to the real world. You should expect to see water where there should be a lake, and grass where there should be grass. Buildings appear to always show up as grass. Also consider that it will not be 100% accurate; in the screenshot, above, the 'water' is actually a children's playground.
As a side note, if you stay in the same area for a while, the ground appears to slowly turn grey and patchy. From my observations, these are 'dry areas'. They show up if you are in the area for a long time, and Pokemon generally do not spawn when you are standing on them.
If you are referring to the blue and green map from a Intel map. It is the previous game niantic released called ingress. That Map shows the playing field of ingress as to who owns the portals, links, and fields. The Intel map is just used to show some pokestops but not all of ingress portals are pokestops but most are. I know because I played ingress for three years. It's really what started pokemon go. Source - ingress agent level 16! Go enlightened.