Wasn't that a Jeff Bridges movie?
Seriously, it seems to be obvious that the dressmaker in question considers color to be a differentiating quality. You seem to disagree.
Are you using the word "custom" to mean "unique"? Men's tailsuits are - technically speaking - custom made but one would have to be something of an expert to distinguish one suit from another.
I'm curious about what others may think. Does a different color or color combination make a different dress?
As to your oringinal question, it appears to me that getting a completely unique design may require spending considerably more money than is customary.
In order for you to be assured that your gown will not be duplicated, you and the vendor will have to come to an agreement over the meaning of the word "custom."
jj