“You're growing up. And rain sort of remains on the branches of a tree
that will someday rule the Earth. And it's good that there is rain. It
clears the month of your sorry rainbow expressions, and it clears the
streets of the silent armies... so we can dance.”
Let's try to approach this linguistically without getting into lit-crit.
To clear something1 of something2 means to remove something2 from something1.
He cleared the table of dishes.
The police cleared the room of people.
Just as rain clears the streets of the silent armies, it clears the month of your sorry rainbow expressions.
The "silent armies" clearly refers to people on the street. The rain makes people go indoors.
But where the relationship of people to street is clear, the relationship of expressions to month is not. Is there a literal meaning to be found there?
expressions can refer to the way a face looks or to things people say.
rainbow as modifier doesn't remove the ambiguity, since it applies literally to neither meaning (facial expression or verbal expression) but can be applied figuratively to both.
The word rainbow when used figuratively can connote optimism. A rainbow verbal expression could be optimistic words. A rainbow facial expression could be a smile.
But these are "sorry" expressions.
The word sorry when used figuratively (when it is not referring to contrition) is a pejorative with a very wide range of meanings: "inane, stupid, of little or no worth or value, misguided, unsuitable, dilapidated, substandard".
He rode a sorry bicycle, its wheels bent and wobbly, one of its pedals
missing, and the rear tire nearly bald.
You call this sorry paragraph a book report?
A sorry verbal expression could be a trite one. A sorry facial expression could be an insincere one. But since the range of meanings of sorry is so broad, all we can say is that the speaker considers the expressions to be of little value.
The rain is good in that it clears away these expressions from the "month", a span of time lasting four weeks :)