Hellow,
I use yellow bulbs and specifically looked for them as you may have seen in another thread. I have chosen yellow bulbs because it clearly distinguishes me. When a driver see's a sea of headlights, I am clearly recognisable.
This has worked well for me as it made a very sensible difference in the times I was being cut of by car drivers. As the bulbs are high quality ones from Hella (with E-mark), they work faultlessly and have an even spread of light and a clear yellow colour (as compared to cheapo Chinese ones of doubtful origin that can spread a rainbow of colours everywhere but where it should be).
This Friday I had to travel a few Km through French countryside at night and both low and high beam were more then up to the task. I could clearly see where I was going, yellow does colour white reflectors....yellow but thats about it. Everything else is clearly visible with the yellow bulbs both in dry and wet conditions.
However if you are not too concerned about being seen as I need to be filtering through Motorway and London traffic, then I would stick to high quality Philips +30% bulbs - no, not the +100% as they just don't last.
Do NOT put any of those blue bulbs in your headlights, as these really do distort the notion of distance of things beside being rubbish in adverse weather conditions. Blue light has a much shorter wavelength, hence stuff looks much closer then it really is. Yellow light does filter out blue light waves and all distances are perceived correctly.
Some more info on the subject:
What is selective yellow:
http://www.danielsternlighting.com/tech/lights/light_color/light_color.htmlLight in general:
http://www.danielsternlighting.com/tech/tech.htmlDisadvantages of HID:
http://www.danielsternlighting.com/tech/bulbs/Hid/disadvantages/disadvantages.htmlThat last link is just in case you are thinking about it.
Regarding number plate colours, I did read a little about that subject, too. There are a few colour combinations that are more visible then others, mostly due to contrast. There are three basic ways of influencing visibility: Shape, contrast and size. I could go deep into details here, but I cut it short to keep on the most relevant information for this discussion. Some background info on that can be found if you search for the Battenburg design like the British Police uses (e.g.
http://en.wikipedia.org/wiki/Battenburg_markings)
The most visible contrast is black on yellow background, followed by green on white background, followed by red on white and only then black on white. Light colours also are more reflective, enhancing safety. An impressive picture that shows this you can see here:
http://www.mahonkin.com/~milktree/hawk/reflector-pics/ (I have Solas tape on the bottom of my forks, my vest and my helmet and red EC104 tape on the back of my top box.
Certainly, the less visible contrast colours are grey on black or silver on black like we see on some show plates some people drive around with. Of course these are less visible and readable and in my experience, people using these plates like to drive like they can't be caught. .