US DOT allows yellow rear turn signals (the so-called 'Euro' turn signals). when they are flashing & the red brake lights are activated, there is less-chance of them being mistaken for front signal lights & there is no headlight between them, which would of-course indicate a bike coming towards you as-opposed to a bike's rear in-front of you.
I like the Signal Dynamics license plate surround w/top & bottom red LED light bars & amber ones on ea. side which you can use for turn signals.