When in doubt, tone it down. Generally, limit the design to include only one or two colors. Too many bright colors can make a design appear garish. Although colors mean different things in different cultures, in many Western cultures darker colors tend to be more formal, while brighter colors tend to be more casual. Darker colors are more subdued and more business-like.
Lighter colors are more festive.
For a formal design, choose darker colors. For a festive design, choose lighter, brighter colors. Yellow accents add life to a design. Yellow can be a difficult color to use because of its strength. As a general rule of thumb, yellow should be used either in small quantities or as the dominating color.