Achromatics were a yellow alien race. Zoe Herriot saw them in a tank. They said people were beautiful, and were essentially programmed to love all humanity. They could change their skin tones. They healed people in close proximity as Zoe's cut was healed when she was close to one. They were based on Dorian Gray's picture, designed with the idea of taking disease and injury from those they healed, but the experiment was currently a failure as the Achromatics took everything from those they healed rather than just their injuries and illness, and would inevitably die of the injuries they absorbed.
The Second Doctor was able to destroy the Achromatics when he infected himself with a fast-acting toxin, the nearest Achromatic taking the toxin from the Doctor but subsequently dying itself before it could take any more from the Doctor, starting a chain reaction as each Achromatic tried to cure its predecessor and was then 'cured' by the next one, until the final Achromatic 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)