Summary so far on how to cope with extreme lighting such as magenta made up of Red and Blue LEDs only.
My personal take is to try to minimize the adverse effect in the picture taken if at all possible - any post processing is kind of like correcting a mistake already made and the damage is already done - but that's just me and YMMV.
However I call it "extreme" lighting - because it is - and it is very difficult to mitigate - at least with my limited experience and knowledge.
No amount of white balance adjustment seems to be able to cope with something like magenta made up of red and blue LEDs only (since the light would lack any green component).
So far I have found using weak fill-in flash seems to help somewhat - as that throws some full spectrum white light into the scene - thus mitigating the extreme lighting.
I have now found this is different for different cameras -
slightly OT - I use a Canon G10 compact far more and therefore have a lot more experience with it - I have a custom setting of "P" with AutoISO, AWB, -1/3 stop overall exposure compensation and slow-sync fill flash at -1 2/3 stop compensation (when flash is fired on the G10 the AutoISO limits the max ISO to ISO250) -
this seems to generally do well - allowing most of the ambient scene to show well but giving just enough flash to mitigate most extreme lighting - the examples shown in Post #
83 above are from the Canon G10 with -1 2/3 stop flash compensation.
So I thought using a -1 2/3 stop flash compensation would work with the K-x too - sometimes it did, but mostly it did not do as well - the results are no where near as consistently successful as the G10.
Pentax K-x settings - "P", AutoISO (200-5000), HighLight Correction On, AWB (w. subtle tungsten correction), -1/3 stop overall compensation.
Through trial and lots of error - a -1 stop slow-sync (first curtain) flash compensation seems to give better results - this could be because with flash the K-x does not drop its AutoISO to ISO250 like the G10 but stays with whatever the shot without flash would have used.
I would have thought any flash compensation ought to be relative to whatever AutoISO chosen - but maybe it isn't - whatever the reason I find I have to use about -1 stop slow-sync flash compensation to get more consistent results.
pudding eating -
No flash -
-1 stop slow-sync flash (all other settings exactly the same)
No flash -
-1 stop slow-sync flash compensation (all other settings exactly the same)
this last pair shows that perhaps -1 stop maybe a little too much - perhaps -1 1/3 or even -1 2/3 may have been better - but there is no way I can tell for sure - however as an overall picture I think it's just better with the fill-in flash.
Sometimes it just doesn't work as well -
No flash -
-1 stop flash
the flash shot doesn't look that much better than the no flash shot -
one of the main points to using the fill-flash is to try to retain details in the magenta light -
No flash detail -
-1 stop flash detail
in this case flash didn't do too well -
perhaps for whatever reason -1 stop wasn't enough - or the K-x over-saturated the red and blue channels, so adding white light didn't help?
Post Processing - I would much prefer to use this as a last resort - kind of like salvaging the shot if I didn't get it right at the taking stage.
To recover the use of select white or grey point in any photo editor seems to do quite well as shown in post #
79 - for more extreme/difficult cases where RAW can't even manage - and i have shown this in Post #
23 - the use of Pentax DCU/SilkyPix 4.11 on either the RAW/DNG or
JPG and using the select grey-point seems to be able to recover the extreme lighting - see Post #
86 for an example on a JPG.