There is no definitive answer to this, all there are is theories.
One theory is that sailors returning from sea painted their houses for maratime purposes.
One is linked to Bristol’s maritime history. It’s said that sailors returning from sea, painted their houses distinctive colours so they could chart home in the fog.
The second theory is that Bristol's previous mayor bought a property in Cliftonwood and painted it red to prevent it from being demolished, though I am unable to provide an answer as to why painting a building red would stop a city council from demolishing buildings.
another popular story concerns our previous mayor, George Ferguson, who back in the 1970s is said to have clubbed together with student friends to buy a house in Cliftonwood. To save it being from being demolished, the story goes, they painted the house red. The house was saved and slowly all the houses in the terrace followed suit.
Another theory is that a painter and decorator offered cheap house painting to make use of paints he had acquired.
Angie Parker Textiles
Another rumour is that this trend started in Totterdown, where a local decorator offered cheap house painting to make use of the free coloured paints he’d acquired.