"The papers must be blue"
Yes, you are correct that this sentence could mean either:
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:
- with "certainty",
- with a strong, determined emphasis,
- with no emphasis, or "matter-of-fact".
On the other hand I would interpret it as a statement of expectation or hopefulness if the word "must" was spoken:
- with "uncertainty",
- with an exaggerated expression of hopefulness
- perhaps with an uncertain facial expression.
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?