Black, navy, charcoal gray with or without pinstripes are the most conservative choices. Although these neutral, dark colors are always safe, you can liven things up with color as long as other respected women colleagues do the same. Red is considered a power color for women; it's not unusual to see women senators wearing red suits. Base other color choices on what looks good on you, not whats trendy.
In your major garment pieces, you should avoid the "baby" pastels such as pink or blue because they may tend to emphasize your femininity, rather than your valuable work skills. If you like to wear soft pastels, pick those hues for your shells and blouses that youll wear with suits.