Agate is a fair bit darker in leather than the platinum grey that used to be available and which I have in my Boxster S and Cayenne S. It can give more of a brown, beige, grey mixed colour appearance than and all out grey. I think there is a Slate/Graphite grey option for leather which does not have the brown tone in it.
As for your exterior colour it is a great choice, I went to see a 718 Cayman S in that colour at Porsche Belfast. It is remarkably similar to Agate from a distance. I think someone on here put up a picture of an agate car beside an aventurine Green car and the difference was only in shade and hue. Close up you can determine more of a difference.
I would agree with the above post that a sandbeige and black interior would look really well. I saw a night blue 718 earlier this week with a two tone, what looked like crayon and black interior, I think the seat centres were in race tex. It looked smart.