Make unimportant things grey
Grey is a very useful color for things that aren't very important. Use grey to color elements that...
- are unselected or backgrounded
- provide context to an important data point
- are not the currently selected element
In fact, let's just make grey the default color. Everything should be grey! Unless of course you'd feel comfortable shouting that data point out at a stranger.
I say err on the side of making your greys lighter rather than darker.
You have a line graph of life expectancy over time for 20 different countries. You're currently interested in India.
Make the 19 countries that aren't India grey, and then India gets to be red or green or whatever your favorite highlight color is.
You can pick your own colors if you're a design genius, but if you aren't just use http://colorbrewer2.org and you'll probably be okay.
And pastels are very in these days, so use them if you want to be popular.