The name Rainbow Bridge was decided by the public, and while the bridge is actually white, some steps were done to create an appearance to justify the name - some poles on the bridge's sidewalk are actually painted in rainbow colors and, at night, there are 3 kind of lights, red, white and green.
However, viewed from the side, the bridge looks green:
EXIF info:
Nikon D90
Lens: 18-70mm F/3.5-4.5G
Focal Length: 46mm
Aperture: F/5.6
Shutter Speed: 3s
ISO Sensitivity: ISO 200