What do the different colors on the map in Pokemon Go represent?

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.