1) His paintings are expressions of his strong personality. Is fine.
2) His paintings are expression of his strong personality. Not good.
3) His paintings are the/an expression of his strong personality. Fine.
If you balance things across 'is' or 'are', the right side renames the left side with a different name.
4) Elephants are big animals. Roses are fragrant flowers. Tea and coffeee are refreshments. Fine.
5) Elephants are big animal. Roses are fragrant flower. Tea and coffeee are refreshment. Not good.
You can directly appose the elements of 4):
6) Elephants, big animals are.... Roses, fragrant flowers are ... Tea and coffeee, refreshments are ....
You can't do that with 5) nor with 2) 2) His paintings, expression of his strong personality are ... , mainly because 'paintings are' 'expression is'.
3) His paintings are the expression of his strong personality. In this sentence, 'paintings' is a plural, and 'the' is used as an adverb, equivalent to something like 'each time', compare this with 'the more the merrier'.
If you use 'an expression' 'His paintings' is as a collective plural noun, viewed as one thing, and made equivalent to 'an expression', a singular noun. This last bit is rubbish, of course, and that was just what you wanted to know!
6) His paintings are an expression of his strong personality.
7) His paintings, an expression of his strong personality are wonderful/rubbish/moving/provoking. Not so good, see 8)
8) His paintings, an expression of his strong personality is wonderful/rubbish/moving/provoking. Not good, see 7)
In 7) and 8) I have done the same as in 6), but 7) seems to say 'an expression are' and 8) says 'His paintings is' Not good.
Since 7) and 8) are not good we have a problem with the interpretation of 'an expression of his strong personality' If it is across from 'His paintings' separated by 'are' and can not affect the verb, then it must be an adjective, a lá 'good'. That's how I interpret it in 6) and 7), also Konungursvias' examples are the same.
These are good examples of malleable English syntax. You probably didn't want to know any of this, but I need to practice sentence analysis, so forgive me please.