Disclosed is a driver that includes a main power supply that has outputs coupled across a string of light emitting diodes to supply a main current. A secondary power supply is coupled to a junction between successive light emitting diodes in the string to supply or withdraw a delta current from the junction. The delta current is at least five times smaller than the main current. A controller controls the secondary power supply to generate the delta current to obtain a desired spectral composition of the mixed light emitted by the string.