Black and blue Pens, mechanical pencils with refills, possibly a couple permanent markets of different colors, all of them in a convenient side pouch on my backpack.
I've become accustomed to using blue, purple, red, green, pink, and black pens.
The colors really make my notes easier to visually scan. Especially if I'm trying to keep a topic on one page and end up filling in gaps with sideways notes or diagrams.