It doesn't matter. I would keep it to black or blue though, preferably black; just make sure it's legible. If your not already, you might want to get used to using a fine point pen. [Later, in professional flying you will be filling out forms, sometimes with small spaces.] Also don't put anything stupid in the remarks column!

Another note about pens: Unless I'm flying Aerobatics, I typically keep a pen attached to the lanyard around my neck, along with a very small flashlight. It's not for convenience, more as back up in case you drop either in flight.

PS, When you get to the point you are flying long days with multiple legs, and don't have time to put them in your logbook, at least make pertinent notes of those flights on a clearance pad or such that you don't forget later.