actually red filters don't "add red".. they actually filter out the blue wavelengths, and the effect is that it will be less blue and it balances the filtering effect of the water which filters out the red wavelengths. due to this filtering effect of the red filter and loss of the blue wavelengths, less available light hits the sensors, so exposure is lower when you use filters compared to no filters. that's why some people complain their pictures are dark when they use red filters.
just like how coffee filters don't add coffee.. they filter out coffee!
This is a good point, the physics behind colour perception can be a bit interesting at times. We're not adding red, but removing blue to make the picture appear more "red". Red filters appear red because there's no blue. Our view underwater looks blue because there's no red.
Thanks for pointing this out.
Reminds me of an interesting piece I read a while back that demonstrates colours can sometimes behave in funny ways. There is no such single colour called "magenta"
- it is a made up colour caused by the absence of green (or the combination of red + blue). Mind boggling stuff.