You only need four pens at all time

3 black pens;
One to use: Self explanatory.
One to lose: Also known as a back up.
One to give away: Because no one else seems to carry their own damn pens.
One blue: For official documents or to impress someone. Blue ink is not as common as black and stands out. It makes an impression and you'll more easily be remembered on resumes, applications, or giving out your number to that hot number.

You could carry a red pen, but traditionally that's for correcting things and is not valid on most legal/official documents