If every region looked the same it would be rather dull but thank fully Cities: Skylines comes with three very distinct climate regions for cities. The three are North, Sunny and Tropic.
A theme includes its own set of trees (pine and conifer trees for north, broad-leaved trees for sunny and palm trees for the tropic theme – there are also some overlapping flora found in all of the themes), terrain colors, water texture and lighting conditions. For example, the sun is lower in the north theme and the grass is darker as well. The water is also a darker blue. In comparison the sunny theme has sun higher up in the sky and the lighting reflects more the Central and South European lighting conditions. The grass is greener and the water texture is somewhat more vibrant compared to the north theme. The tropic theme emulates tropical island atmosphere with sand texture that has a nice, slight glow to it when the sun hits it.