I'd say the red PTD (shiny side up, of course) would filter out the red spectrum of
light and the gree would filter out christmas trees.  I would advise you to ensure
you have at least a 1" shiny aluminum strip between the red and green (to hold them
together)...  ;)

Chuck Hill wrote:

> Joel Walker advises us:
>         "I wear 2 lbs of aluminum foil (arranged properly in a toroidal hat) on
> my head at all times during the night (when the ozone layer is at its
> lowest point!!).  i can tell you that without that aluminum foil i'd be
> receiving those messages in my teeth and brain,......
>
>         Joel forgot to mention the most important part of this safeguard:
>
>                    KEEP THE SHINY SIDE OF THE FOIL UP!!!
>
> If you do this wrong, and make your PTD (Protective Toroidal Device)
> with the  dull side of the foil up, the effect is one of concentration,
> instead of reflection/dispersion, leading to dire consequences.
> Absolutely no dull foil must face upwards.      KEEP THE SHINY SIDE UP!!
> The PTD offers pretty good protection during periods of low ozone
> protection, but is less satisfactory during daylight hours, the
> reflected/dispersed rays causing discomfort to those you may meet on the
> sidewalk.  I have noticed people actually crossing to the other side of
> the street downtown while I've been wearing my PTD during periods of
> "holes in the ozone" during daylight hours. Sometimes during evening
> hours as well, though not so often.  People do know about the benefits
> of the use of a PTD, and though you don't see a lot of them in use, I
> notice people pointing and nodding their heads in approval rather often.
>
>         I would be interested whether red or green colored foil is more or less
> effective--perhaps someone can report on this to the list.
>
>         A PTD is also a good precaution during periods of high sunspot
> activity, though there is some mention in the literature that pyramidal
> shaped devices are just as effective.  I notice about the same reduction
> in message reception through the teeth and brain with either one, so I
> just stick with the one PTD.  The pyramidal device is rather flimsy,
> also.
>
>         Those readers in the southern hemisphere should realize that in their
> case, the dull side should be up.  It's reversed, just like the Coreolis
> effect.
>
> Obligatory Vanagon Content:  I always keep a PTD in place on the engine
> cover during dangerous periods, and have had no more problems with
> surging, stalling, etc. since using the device.
>
> Chuck Hill