Hello:

Which color should I mix with Waterman Havana to make it a darker, more "chocolate" brown. Tried Black but it didn't turn out to well. Any suggestions?

Mescoff

I haven't tried any brown mixes, but maybe green (if you already have some green, don't go buy any on my suggestion) might get you toward where you want to go.

I bet some green and a darker red would do it (like Fiesta Red). It works in my imagination anyway