Cheil's Tim Polder has created a tweet powered-sculpture that comprises a rainbow composed of layers of liquid separated naturally into individual colours.
People who tweet #PutRedBack campaign trigger a single drop of red coloured liquid into the clear part of the rainbow.
The activity has been created as part of some pro bono work for charity Freedom to Donate and aims to raise awareness of the restrictions on gay men donating blood.
The installation has been placed at the London Design Festival to help "shift the debate" forward.
The organisation needs 100,000 tweets to "put the red back".
The project is being displayed at Somerset House until 27 September and the charity is also calling on people to sign a government petition that will lead to a parliamentary debate on current blood donation guidelines.
Brand: Freedom to Donate
Agency: Cheil UK
Creatives: Tim Polder and Matt Pye