Portal/colour-blind girl recovers/green jewel/accused of faking

This is an older children's or young adult's book that I probably read in the mid to late 1970s. There are some children with various disabilities and I think they may all live in a children's home, or else go to the same special school. There is some kind of portal that they go through, but I don't remember what they do on the other side.
There is one girl who was colour-blind (not red-green colour-blindness as she could only see black, white and shades of grey), and when she comes back through the portal she can see colours for the first time in her life. One of the first things she sees in colour is an emerald ring that her psychiatrist/doctor/guardian? (definitely not her mother) is wearing and when she says how beautiful it is, she accuses her of having faked her colour-blindness. At the time I thought how ridiculous the accusation was - how could a child have faked it her whole life?
The other children were helped by going through the portal, but I don't remember any of details about them.

Did it mention the word achromatopsia (inability to see colour) in the book, do you think? Hmm! A book I don't know... sounds interesting.

I don't remember it mentioning the word achromatopsia, just saying that she only saw in black and white.

Someone on the LiveJournal group whatwasthatbook has suggested that it could be "The Magic Meadow" by Alexander Key.

4: It's definitely not The Magic Meadow with the colorblind part. The earlier premise (group of children with disabilities) matches The Magic Meadow though, which is an excellent book.

Finally found after re-posting it on the LiveJournal What was that book? community.

It is definitely The Fair to Middling by Arthur Calder-Marshall.

Oh wonderful! Now I can wishlist it!

I have just re-read The Fair to Middling and it's still a good read. I probably remembered the parts about Emma (the colour-blind girl) because she is the one who was changed in the most obvious way by her experiences at the fair, but it could also be because she is a girl or because she had red hair and I always wanted to be a redhead.