Summarize the following:
The point of adding color to your poster is to make it visually appealing; it should draw and audience. Too much color is confusing. One or two accent colors that are eye-catching and emphasize your subject is all you need. If your poster is themed use a matching color. For instance, if you're doing a presentation about breast cancer, be sure to use the correct pink. The audience will see this and be drawn to it because it's familiar. Use a poster that has a light colored background and a dark colored text. This not only saves a tremendous amount of ink, but makes it easier for your audience to read.

Summary:
Make it visually appealing. Understand the message and audience. Use dark colored font.