Usefulness of a headlamp whilst hunting depends very much on what you want to use it for. If it's for general use, looking in your bag for bits and bobs, skinning or cleaning game at the end of the night and this sort of thing, fine. If you were thinking of using it as a hunting lamp, to actually locate and shoot prey with, I'd say forget it. A friend holding a decent lamp, or a rifle mounted light (Like the ones from Deben
http://www.deben.com/lightsandlamping/tracergunlights.htm ) will be far far easier to use. Trying to align rifle sights AND a head torch is a nightmare. I tried it once, and that was enough to convince me to invest in a proper rifle mounted lamp.
Red filters are available to fit the rifle lamps and are made of proper filter material. I've tried all the dodges, red marker pen, red plastic from inside biscuit tins etc etc, and nothing gives the pure red light that a proper filter does. These days I use red "gel" (actually a thin plastic available in more colours than you can name) as used on theatre lights to convert any light source I want to a pure red light.