I found this thread over at CandlePowerForums, and did some experimenting (basically, the swatches are samples of photographic gels, 1" x 3", that are used in movies and photography to balance the color or temperature of source lighting). For flashlights and headlamps, the typical cool-blue tint of LEDs falls short outdoors, as browns and grays tend to lump together visually to some. Warmer LEDs (non-existent in the mainstream manufacturers like Petzl, Princeton Tec, and Black Diamond, etc.) make these colors "pop," often improving visual clarity on the trail. Of course, YMMV.
All I can say is WOW! I took my c. 2007 bluish Fenix L0D and fitted a tiny circle of #3409 (a pinkish, beigeish color) over the front, and now I have a near-perfect cream white color light that rivals my c. 2011 "neutral" Quark AA XML. I have since "balanced" my c. 2008 Fenix LD1 and my "aging" Zebralight H30. (For the H30, I chose a slightly warmer #3408 gel since I use that more outdoors.) I can't wait to "balance" my Princeton Tec EOS, and my rather ugly NiteIze TaskLit (0.9 oz headlamp).
Adding a filter works (so far) best for single-LED lights; I'm not sure how these filters would fit over a Tikka-style lights. (For the Tikka lights, removing and tracing the clear filter might help.)
Note that adding a filter always lowers the amount of transmitted light, typically 10-15%. But contrast this with the warmer LEDs: they always have lower lumen levels anyway, so it's about a wash in the end.