If you're bothering to hack your torch, you could certainly wire it with a low resistance and a high resistance selectable by the slide-switch (for two brightness settings), but why not:
a) snip the resistor to the white LED so that it won't turn on accidentally, and
b) insert a variable resistor in series for the red leds, so that you can dial the brightness up or down to your taste.
If you're electronically-challenged, I just use one of these on a cord around my neck:
http://shop.australiangeographic.com...3l4xoCtTbw_wcB
It is red only, so no worries about accidentally dazzling yourself, and has two brightness settings, selected by a simple thumb-clicker - one click turns it on bright, a double-click turns it on dim, and a single click turns it off from either mode. (It also has flashing and signalling modes, but I don't use them.)