The blues in Picasso's painting seem to communicate his sadness. It makes me a little sad, too.

Question

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Q1: Do we have to use the plural form 'blues'?
Q2: What does 'It' refer to in the second sentence? Does it refer to 'The blues' or 'Picasso's painting'?

Answer 1

Yes, because the reference is to different shades of blue.

I'd say "it" is referring to "sadness" in the previous sentence.