Our comic is mostly black and white, but during a particular section we added red for blood/glowing stuff, because we really wanted it to stand out. It would have been a lot harder to get the same feeling across otherwise, I feel, so there are definitely times when I'd recommend it.
Though one thing to consider (which was a big thing for us) is that if you want to get it printed eventually, most printers will charge you the full price for colored pages even if only a few are color (unless you have few enough for them to count as "inserts"). Luckily we found Keness, which appears to let you state which pages are color vs b/w and adjusts the cost accordingly, but most places won't do that.
If you don't plan on printing copies no worries then. 