I love this coin. This is the type of toning I really appreciate. And yes that is the actual color guaranteed natural.
Mint set, I assume? Based on the obverse being so well-toned and the reverse getting that golden color
Here is a photo of a mint set toner I won in a giveaway, and sold for $100 in San Francisco last July. It sold for like $40 in auction not long after at GC. This is why I think it was a mint set toner. Since the toning is on the same side on yours and 'mine'. color doesn't show up well in this pic.
No. A 64 would not have the same sort of toning as your 58. In 58, the mint sets were shipped in cardboard holders, hence the toning (from the sulfur in the holder). Mint set toning is characterized by the "speckled" feel that yours shows. In 1959, the mint sets were changed to plastic (cellophane). Anything after this will not demonstrate the classic "mint set toning" that the late 50's are famous for.