1. Look around and decide what you like.
2. Be open to new ideas... you might have something in mind but something better might come along.
3. Decide on the FEEL of your party.
Below is an example of a color scheme we put together for an upcoming event... it's a retro Vegas theme. Picture black vinyl records, mellow blues and pops of fierce reds. It's gunna be fun!
Here's a trick, find an image that you like and create a color scheme from it. This way, you won't get hung up on what you are seeing in the photo, but rather letting your creative juices flow.
We make our color schemes in photoshop, but there are some easy color scheme generators out there. The easiest I have found it Chip It by Sherwin Williams. It's meant for choosing paint colors but it can also be fun to play with.