Buy a 3mm red led from lsdiodes.com for 50 cents or so. Use a 5/32" drill to enlarge the hole in the minimag reflector slightly so the led will fit through it, and use a nail file or sandpaper to smooth down the epoxy skirt at the base of the led. Put rechargable nimh cells (1.2 volts) into the minimag so the lower voltage won't kill the red led. Stick the led into the socket where the bulb normally goes. If it doesn't light, you've put it in backwards, so turn it around.
Also, you can get red lenses or filters from Mag as standard accessories. The red lens replaces the normal lens and the light otherwise looks the same. The red filter kit is a sleeve that slides over the front of the light, making it a little bigger, but that makes it easier to put on and off. Efficiency does suffer.
Finally, you can get a red-led Gerber/CMG Infinity Ultra which runs on one AA and is a nice little light, $15 or so from batterystation.com. The white-led version of the same light, or the hard-anodized Ultra-G version, are among the perennial favorite lights around here.