I’m looking for a book I had in my early childhood – I was born in 1984.

There is a flour mill with a grumpy Miller, and a group of children who go into the mill. While they’re in there a rainbow comes in through the window and ends up divided into a paint pot for each colour. The children take the paints and try to do good deeds – for example, painting someone’s door? sunshine yellow or someone’s window frames? cornflour blue. They paint something belonging to the Miller purple and it matches his angry face.