We report on the development of a novel electronic vote machine interface, with emphasis on the requirements engineering process. In particular, we review how we followed an operational prototyping approach in order to gain a better understanding of requirements in an incremental fashion. Our most interesting observations are concerned with the evolution of our most fundamental requirement: that the voting process followed by the voter should be just like paper. We comment on how the weakening of this requirement was deemed necessary by the addition of other requirements that were identified during our prototype evaluation. This weakening was minimized through the specification of a passive voting protocol that provides feedback to voters without obliging them to follow a voting process any different from that which is normally done using a traditional paper vote. The protocol is based on a simple 3-state machine where we naturally represent the states using the familiar traffic light colour scheme: thus the interaction between voter and interface became known as the three colour protocol.