Paraphrasing someone's question back to them; rephrasing it as a rhetorical question

I think your first example is correct.

You are very welcome.

To complicate matters, I think the second example is correct. I thought the rule was that a full sentence following a colon is capitalized.

In both sentences, remove the comma after "but."

To confuse matters even more (yay), it may depend on if you are British or American. It also depends on the style of the publication you are writing for. Here's a Wikipedia excerpt that accurately explains the issue:

Use of capitalization or lower-case after a colon varies. In British English, the word following the colon is in lower case unless it is a proper noun or an acronym, or if it is normally capitalized for some other reason. However, in American English, many writers capitalize the word following a colon if it begins an independent clause (i.e. complete sentence). This follows the guidelines of some modern American style guides, including those published by the Associated Press and the Modern Language Association. The Chicago Manual of Style,[5] however, requires capitalization only when the colon introduces a direct quotation or two or more complete sentences.[6]

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Here's one way to write your example:

But you asked a very specific question: "What can we actually do to prepare?"