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The papers must be blue
This sentence means: I have not seen the papers but I'm sure they are blue.
If it's an instruction, it means: You must use blue papers. Any other color will not work.
Am I right about the double meaning? If yes, how to solve the confusion?
"The papers must be blue"
Yes, you are correct that this sentence could mean either:
I have not seen the papers but I'm sure they are blue.
You must use blue papers.
A previous answer correctly advises that context is the only way to tell them apart. That is true if the words are written or spoken, but there may be other ways to tell and to express the difference if you were the one saying or writing it.
It is of course quite hard to explain the nuances of spoken expressions, but if spoken I would expect there to be different emphasis on the word "must". I would interpret it as an order if this word was spoken:
On the other hand I would interpret it as a statement of expectation or hopefulness if the word "must" was spoken:
In written form some writers may add italics to emphasise words, so you may see:
The papers must be blue.
But this doesn't really differentiate between the two different types of verbal emphasis I detailed above.
An exclamation mark may also be used to denote emphasis, but again this could be ambiguous. The sentence is not really a question with either intended meaning because it does not solicit an answer, so either way it could be an exclamation. A good novel writer would add some description to the way a character was speaking so the reader could understand the intended meaning, and it is possible that in creative writing a question mark would be added, even though not technically a question, because this denotes the verbal emphasis in a creative way.
A sure way of expressing the sentence to denote the uncertainty would be:
Surely the papers must be blue?
Context.
There is really no other way.
Both your examples work, for a given context.
The papers must be blue.
It shows confidence, yet, consciousness of a degree of uncertainty, in parallel.
The verb:
to be
softens the modality of auxiliary verbs. Their meanings become synonymous.
can be, could be, may be, might be, should be, must be.
Instructions are not usually given using the passive voice. You should use an active or imperative sentence, for more efficiency:
You must use blue papers.
Use blue paper!
These sentences are just given as examples, and could be (may be, should be, must be) out-of-context.
