The Tin Forest by Helen Ward

In the middle of a wide, windswept place inside a small house with small windows lived an old man. He looks out on other people's rubbish and bad weather and dreams of a better place to live.
He comes up with the idea of transforming everyone's junk into trees, birds, and wildlife. One day a real bird appears, drops seed and plants and trees grow. His world is transformed as wildlife comes and the man is no longer lonely.

As his environment changes the illustrations also are transformed from a drab and dismal grey to a sea of glorious colour - very clever and effective.