Achromatics were artificial being created in the Whitaker Institute with the purpose to cure sick humans. In order to want to absorb human diseases, they were programmed to love humans and try helping them. However, instead of only absorbing the sick cells, they absorbed everything killing the human patients they were intended to cure.
Their name was based on the novel The Picture of Dorian Gray by Oscar Wilde, where a portrait was getting old instead of Dorian Gray himself. The purpose of Achromatics was to get sick instead of humans, and the word "achromatic" meant colourless, in other words, grey.
While travelling with Zoe Heriot and Jamie McCrimmon, the Second Doctor was able to destroy the Achromatics by infecting himself with a fast-acting toxin. The nearest Achromatic took the toxin from the Doctor, got sick before it could kill the Doctor, causing another Achromatic to absorb him completely, getting sick himself in the process. The resulting chain reaction killed all Achromatics but the last one, who died of the toxin.
Years after her departure from the Doctor, the Company responsible for their creation attempted to trick the now-amnesic Zoe into recalling the genetic code of the Achromatics, but she recalled enough of the original adventure to refuse to give them such information. (AUDIO: Echoes of Grey)