1. A flash accessory for use with a smartphone, the smartphone being equipped with a camera for capture of a multi-frame video sequence, the accessory comprising: a housing including a portion adapted to engage a portion of the smartphone, thereby enabling the accessory to be removably attached to the smartphone, the housing containing: plural light emitting diodes having different spectral characteristics; an interface adapted to receive a frame timing control signal from the smartphone; and drive circuitry coupled to said interface, and configured to independently control said plural light emitting diodes; wherein the drive circuitry is adapted to respond to the frame timing control signal to controllably illuminate different ones of said light emitting diodes in a programmed sequence, at times corresponding to captures of different frames of a video sequence by the smartphone camera.