Your question touches upon a few areas and you would do well to search the Avvo questions/answers area as a similar question was posted a few months ago and received several good responses. While there is no copyright in dress designs (as they are considered functional items) Disney likely still owns the trademark to the character design of its paricular version of Snow White. (the general story and character name of Snow White are in the public domain at this point) If your design drifts over to the point where it may confuse consumers into believing that it is licensed by Disney, then you are treading in hot water. Disney is one of the greatest and fiercest enforcers of its trademarks. Your patterns may not be protected by copyright, but if you can license them instead of outright selling them to limit their use by the purchaser. I will end this answer here, without going into greater detail, with mystock closing response: If you intend to start a profit-making business, particularly if that business model is based on the use of someone else's intellectual property, it makes sense to budget some of your start-up money for legal consultation at the start as opposed to having to budget for litigation advice later on.
The following is not legal advice and should not be relied upon to take or refrain from taking any action.
To add to Oscar's spot on answer:
The story of "Snow White" and the Snow White character are in the public domain (i.e., no longer copyright protected) because the story has been around for a very long time. See . Which is why there are so many variations of Snow White dolls, books, plays, etc. for sale.
However, while no one can now own the exclusive rights to the Snow White character or story, new works that are created based on the Snow White character and story CAN be protected by copyright by adding unique and sufficiently creative elements to the underlying character and story. The "thin" copyright that attaches to each particular version provides its creator with the exclusive rights to reproduce and distribute that particular version.
The version of the Snow White story made famous by Walt Disney, for example, gave names to the seven dwarfs so the copyright that attaches to that version provides Disney with the exclusive rights to sell that version and all other "Snow White" works with particularly-named dwarfs. Likewise, if a Snow White character contains unique and sufficiently creative characteristics (a particular appearance perhaps) then copyright will attach to that particular version of the character.
As for the "Snow White dress" pattern that you want to sell, you would have to determine if the dress' style and its blue, yellow, and red color scheme is how anyone else has already portrayed the Snow White character. It sounds to me like it's the Disney version. If so, Disney owns the rights to that particular Snow White character -- rights that includes the exclusive right to sell "derivative works" of the character such as the pattern of her particular dress. So, before you begin selling your "Snow White" dress pattern you need to show it to an intellectual property attorney and ask whether it is substantially similar to one that an already-existing Snow White is known to wear.
For the answers given to a very similar question see the Avvo database at:
One issue that is not addressed in Oscar and Daniel's response is that, while the story and film of Snow White and Seven Dwarfs is in the public domain and no longer entitled to copyright protection, the phrase "SNOW WHITE" is a registered trademark of Disney Enterprises, Inc., Serial Number 77098500. That registration applies to International Classfication of Goods Nos. 18, 21, 25 and 32 (they also later registerd the mark under IC41 for entertainment and educational services). IC 25 applies to the use of the mark in connection with clothing. Furthermore, you should be aware that Disney still has several enforceable versions of the Snow White character, even though the original story is in the public domain. You should definitely consult with an intellectual property lawyer in order to determine if your use of the phrase SNOW WHITE in connection with the sale of dress patterns infringes upon Disney's trademark rights or any other intellectual properties.