I think you are confused Henry, this has nothing to do with dialect, and 'which' doesn't necessarily imply choice. The use of 'which' is correct due to the known quantity of options for the answer.
Here are some sources you may enjoy reading:
'Before nouns what and which can be used interchangeably to ask questions about people or things'
This source is particularly relevant:
'You can use which when you have a very small or limited field to choose from. Certainly use which, not what, when there are only two choices, or if both speaker and listener can visualize all the items under consideration:
For example: "Which foot did you break?"'
Another example: 'which colour is the apple?'
In summary, it is correct to use both what and which, though it could be argued that which is perhaps the 'more correct' translation due to there being two colour choices.