(a) That's right. Under the sections of the sale of goods Acts which require goods to be suitable for the buyer's stated purpose, a specific note is made. The sale of goods legislation says that if a contract is to supply a specified article under its patent or other trade name, there is no implied condition as to its fitness for any particular purpose. See a list of the relevant sections.
However, this rule applies only if it is clear from the known circumstances that, when purchasing goods by reference to their trade name, the buyer was not relying on the seller's skill and judgement to supply suitable goods, or if it is clear that the seller is disclaiming any such liability. But if a buyer does indicate to the seller why the goods are required and that they are relying on the seller to provide suitable goods, and the seller in effect says: 'Well, X brand is what you need' then an implied term of suitability does arise, even though the goods are bought and sold by reference to their trade name.
In the example, A makes it clear why she needs the paint and that she is relying on B to supply a suitable product. Therefore, even though the goods are bought and sold by reference to their trade name, there is an implied term that the paint is suitable for the buyer's purpose.