This probably varies from state to state but this is what is listed in the Oregon motorcycle handbook for 2011XVS13CAL said:
Required Motorcycle Equipment
All motorcycles and mopeds must have:
• At least one but not more than three, white headlights. Modulating headlights
are allowed during daylight hours. Oregon law requires that the headlight be
on at all times.
• At least one red taillight.
• One white license plate light.
• At least one red brake light.
• Amber turn signal lights if the motorcycle was built after 1972.
• A red reflector on the rear.
• At least one rear view mirror.
• One horn.
• Fenders on all wheels.
• At least one brake operated by hand or foot.
• An exhaust system in good working order and in constant operation (or, in
the case of mopeds, when the engine is running), which prevents the vehicle
from discharging any visible emissions and keeps exhaust noise levels at or
below standards set by the Department of Environmental Quality.
• All lighting must be DOT compliant.