The advent of light-emitting diode (LED) light sources has led to a new freedom in the design of light-source spectra, and it is now possible to optimise for different source performance parameters, which is the principal aim of the authors’ work. LEDs and lasers are real or potential light sources, and are inherently monochromatic, that is, narrow-band sources, with typical optical bandwidths in the range 20–40 nm (nanometres) for LEDs and 1–5 nm for diode lasers. Mixtures of three or more can be used to produce nominally white light of the type acceptable for general purpose lighting. It is a characteristic of all types of sources that there is a trade-off between good colour properties and high efficiencies, and the methods described here are directed towards an optimum combination of such parameters. This chapter will explain the use of differential evolution (DE) as a highly effective heuristic approach to optimisation, and proceeds to explain the structure and operation of a DE algorithm designed as an optimisation tool for such purposes.