Most here kind feel matching your helmet to your bikes color is ghey...but I won't go there.
But I will go here: (Sorry about reply'n to my own quote)
If you wear a helmet that matches the paint on your bike, you are in essence making yourself blend into the bike visually. It has already been shown that on a subconsious level, car drivers will "dismiss" motorcycles because they are not seen as "threats". They have been trained from day one to look out for two sets of headlights...not one by itself.
Some recognise motorcycles , but do not atatch a human element to it...because the riders often do not look very human.... so it kinda makes sence that if you can make the rider stand out some from the bike they will draw more attention from the inattentive driver.
By "matching" the bike you tend to blend into it, and those other drivers do not percieve that there is in fact a person there. However by wearing a contrasting helmet color, you make yourself stand out more.
It is also true that the human eye can pick out
movement faster if one of the objects stands out from the other that is near it....meaning seeing a white helmet on a blue bike you will detect movement easier than if the helmet was the same blue color. The riders head does tend to move in relation to the bike, especially when viewed at an angle , and many sit up straighter approaching an intersection.
Am not really sure how much a difference all this really makes but anything that gets others on the road to see you is a good thing.
KM