Ms Black, Ms Blue and Ms Green - Is there really a unique answer?

Solution 1:

As requested in the comments:

Gardner's version of the puzzle ends with a line which is omitted here: $$\text {"So what?", said the lady in black.}$$

Note: a reference can be found here.

Somewhat surprisingly, that changes everything. It adds the information that Miss Blue is not wearing black (as she wouldn't have replied to herself, nor would she have disparaged her own observation). That makes the solution unique.

Note how delicate this information is. Had the line been spoken by Miss Black, we'd have learned nothing. Had it been spoken by "the lady in blue" we'd have learned nothing (as we already knew that Miss Blue was not wearing blue).

Solution 2:

There are two options not matching. If they sit in a triangle wearing their own colours, they can rotate the dresses once and then again, both of these cases nobody wears their own.

'"So what?" said the lady in black.' Indicates that the lady in black is not Miss Blue, leaving only one possibility.