Keeping it simple to two to four colors is best. Using different shades of the same color is always pleasant and a safe way out of the conundrum. But those that really want to impress, adding distinctive color contrast to your poster will help it stick out even more. Use color in images, graphs, diagrams and to highlight different sections of your poster. Try not to use different colors in your text, instead use text modifications like bold and italics to emphasize which information is more or less important.