There's a film opaquing pen made, used for the same purpose. I have several at the studio, I'll look who makes it when I'm there tomorrow. Works like a champ, less than $10 and comes with an extra nib as well.
A good quality water colour paint like Winsor Newton's will do the same job. Red Ochre is the best colour. One thing: dilute it well as the paint has glycerine in it and will not dry well on film if it is too thick.