I'm always tickled pink to be able to help people identify books!
Your book is "The Great Blueness and Other Predicaments," by Arnold Lobel.
"A wizard color by color retints the black and white world by
inventing a paint that makes everything that shade. Too much of a good
thing it affects mood and creates conflict(red) , sadness (blue) so
on... the 'people' for whom the wizard works in the story demand the
color changes, get their fill, demand yet another color. Rather like
humanity discovering, gorging, purging...at some point an accident
resolves the story and I hope you'll forgive me for this...all color
is spread around and harmony is found in the mix."
Amazon: The Great Blueness
"The great blueness and other predicaments by Arnold Lobel.
New York: Harper & Row, 1968.
When a wizard discovered that each color he invented for the colorless
world had a different emotional effect on people, he luckily had an
accident which resulted in red apples, green leaves, and yellow
If you're looking for a used copy, this may help:
AddAll Rare, Used and Out of Print Book Search
My Google search strategy:
Google Web Search: wizard "the great blueness"