The city above was created by using the Perspective Grid Tool in Adobe Illustrator. In fact, a city is the easiest thing to create if you have enough time. You simply apply the Perspective Grid Tool and mess with the anchor points for the grids so that they look like you want them to. Then, you simply begin drawing the rectangles for your buildings. As for the colors, they were taken from gradient libraries in Illustrator.
Below is another version of a city created with the Perspective Grid Tool but in that one I used Pantone colors for the buildings instead of the gradients.
One thing to always remember about perspective pieces is where the light is coming from and what will be shaded. After all, all the city is is rectangles drawn onto the Perspective Grid Tool.