Could also use a Sharpie Paint Pen. NOT the regular ordinary Sharpie. Sharpie also makes a paint pen that has oil based paint in it.
An extra fine point or a fine point would probley work for your project.
I use them all the time in both of my hobbies and prefer them over regular Sharpies but that's just my opinion
Oh yea they also come in quite a few different colors and in several tip sizes.
Can be some times hard to find in stores, I always order mine on line. Blick Art Materials carries all of the colors and are really easy people to deal with.
Hopes this helps
Jeff