1. Does this "turn sour" mean "become less enjoyable"?
2. Does this "tone" denote "color" or "atmosphere"?
st44)We delight in acquiring new things, but seldom do we consider the resulting ripple effect described by the French philosoper..he wrote about receiving as a gift a beautiful scarlet dressing gown, then quickly discarding his old one. But as he began to sense that his surrounding now appeared worn and unworthy of the magnificence of the new garment, his pleasure turn sour. As Juliet Schor explains, he grew dissatisfied with his library, the desk, his chairs, and even thre room's bookshelves. One by one, the familir but well-worn furnishing of the library were replaced....Diderot found himself ...regretting the work of 'overbearing scarlet robe that forced everything else to conform with its own elegant tone.
To "turn sour" means a bit more than "become less enjoyable". Simply becoming less enjoyable could mean that it was still enjoyable, but not as much as it had been before. If something is said to "turn sour", it means that it is not at all enjoyable any more - even that it has become repugnant, or unpalatable.
In this context, "tone" refers to both the colour [BrE spelling] and the style - and the whole atmosphere it engenders. He uses the term "overbearing" to give the same impression.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....