The emission colour of semiconductor quantum dots is generally controlled at the synthesis stage, by their size or composition. However, being able to tweak their colour post-synthesis would enable devices such as tunable quantum-dot-based pixel displays and lasers. Although individual quantum dots are known to shift colour in response to electric-field-induced charge separation, the effect is minimal, and the intensity also drops. Uri Banin and colleagues show that a ‘quantum dot molecule’ — composed of two fused and electronically coupled CdSe–CdS core–shell quantum dots of different sizes — can reversibly switch between two colours up to ~50 nm apart over most of the visible spectrum, without intensity loss. Because the electronic wavefunctions of both quantum dots are delocalized in the coupled ‘molecule’, an applied electric field can freely toggle an electron between the two emission centres. Appealingly, the concept is chemically modular: the specific emission centres of the ‘molecule’ can be easily adjusted by the size, composition and perhaps even the number of the constituent quantum dots.