Right - I've been out playing with LEDs, and now my hands are just warming up enough to type!
The conclusion is... you definitely need
RED LEDs behind your red filters.
Whilst the two LEDs I was comparing are not identical - one has four large white LEDs and the other has 6 smaller red ones - the white LED is a seriously bright beastie. But behind the red filter it was truly gutless. I suspect this is because the white LEDs are "cold" - so they're biased towards the blue end of the spectrum, which of course is getting blocked out by the red filter. So the physics textbooks are right - not surprisingly.
You do end up with a much deeper, richer red from the red LEDs. It's a lovely colour, but it's not a good match next to conventional bulbs. There's no doubt you'd want to change all four tail lights eventually, though it doesn't look too silly because at least it's symmetrical.
The red one I used tonight was one of the first ones I bought. It's a directional one with 6 LEDs pointing backwards. Looking at it again, I still think the light distribution is not ideal but it is certainly bright enough. You do get
some illumination of the reflector though, so this bulb would be adequate.
The last product I want to try from ultraleds is the
Red 8 Led Super Wide Angle Led Bulb. This is the 19mm wide one, but there is a chance I might be able to jiggle it in, or failing that I could cut/drill/file a pair of notches in the hole in the reflector so that it fits. I'm reluctant to spend any more money though, so I'll hope that ultraleds will exchange some or all of the red ones I've already got.
