One of the biggest choices you'll make when building data visualizations is choosing which colors to use to represent what. While it's certainly possible to convey a lot of information through purely monochromatic charts, using the wide range of color representable through a digital display can be a very effective way of depicting another dimension of the data you're visualizing.
If you plan to use color to communicate information, there are a few things to consider. The first is whether a user will be able to discern the pattern necessary to understand what the colors mean. If you have a legend explaining what color corresponds to what, try turning it off and thinking about whether you're still able to understand what the color combination means. Is it intending to show increasing intensity, diverging values, or just that it has a particular quality (in which case, you'll definitely need a legend)?
Secondly, pay some attention to people who are colorblind. The most...