Yes, you could.

[sitifan]

We do not use could to give or refuse permission.
--Could I ask you something?
--Yes, of course you can/*could.
(Practical English Usage by Michael Swan, Third Edition)
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--Could I see you tomorrow, then?
--Yes, (I think) you could.
(English Grammatical Structure by L. G. Alexander)
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In dialog #1, could is wrong. In dialog #2, could is right.
Why the difference?

 

[Rover_KE]

In dialog #1, could is wrong.

It doesn't say could is wrong.

 

[jutfrank]

It doesn't say could is wrong.

That's exactly what it says.

In #1, could is used only for politeness, so there's no real sense of possibility, as I think we're supposed to imagine in #2.