Try a Muted Color
For the office and for occasions on which you don't wish to stand out, wearing more muted, monochromatic or neutral colors works best. Monochromatic colors are the colors that are different hues of the same color on the color wheel. For charcoal pants, this means to go a shade darker (black), or a shade lighter (light gray). Neutral colors are those that can be paired with anything; a white or ivory shirt will work as well as a black or gray one.
Stand Out with Bold Colors
Since charcoal is a dark base, any bright color will work. A turquoise or fuchsia-colored shirt will make you pop in a more casual situation or in a season like spring, where bright colors are more commonly seen. If you don't want to be quite so noticeable, a paler pastel color will also stand out nicely against charcoal pants.
Charcoal is a nice neutral base color. It can be worn with pretty much any color in any season. What you wear is mostly about your personal preference.