Bizarre early 1970's children's picture book about life in prison

Question

My friends look at me askance when I tell them about this surreal children's picture book I read in a US public library about 1975.

A man (who my brain insists on identifying with Ned Riddle's newspaper comic strip character Mr. Tweedy⁽¹⁾) visits a prison. I can't recall if the "visit" is as an inmate, a consultant, or just as a visitor.

In the book, the prisoners are all very sad because they spend all day breaking rocks into smaller rocks, and mixing different colored paints into something that invariably turns out dull grey, and as a result all the walls are always dull grey.

Our hero suggests to the warden that they use the colorful paints without mixing them into grey goo.

We end up with a prison whose walls are different bright colors and the prisoners are all much more cheerful.

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⁽¹⁾ This was long before the Chicken Run guy.

Answer 1

123456789 Benn by David McKee perhaps.

From the LoveReading4Kids review:

Award-winning David McKee’s highly original character Mr Benn enters a fancy dress shop on a whim and chooses a black and grey costume with the numbers 123456789 on it. No sooner has he tried it on than he finds himself in…a prison! Everything in the prison is grey and gloomy and everyone in the prison is sad. They are so sad that they spend most of their time crying! Mr Benn can understand that being locked up makes people sad but he finds it hard to know why the men all cry so much until he meets Smasher Lagru, the self-proclaimed boss of the prison, who explains that it is the greyness that gets everyone down so much. What can be done? Mr Benn comes up with an audacious plan and soon the prison is transformed into a place full of all colours of the rainbow. And everyone is very much happier.

It occurred to me that I had seen the book when doing some librarian work on goodreads.com for the author's listings and checking further it seems a possibility.