If it was out in the cold, the plant was most likely dormant. It may take it a while to wake up indoors.

FataMorgana

Its not a dying plant though, right?

When it recovers, it will have a strong chocolate mint smell and taste like it should?

It's clearly not dying. I'm wondering if the colouration is due to poor nutrition. I'd try repotting it.

The chocolate mint taste/smell seems to vary widely with individual plants, growing conditions, and even those doing the tasting/smelling. Short answer, it may develop that smell/taste for you. Time will tell. Be sure to read back posts here. This has been discussed many times.

FataMorgana

Oh yeah, purple color is common in cold plants. My lemon balm turns purple late fall. The thyme is purple all winter. Etc.... You can also look through back posts for this too.

FataMorgana

Give it some more time. But later on if it doesn't develop a chocolate-like smell even when a leaf is crushed, and doesn't green up a little, take it back to the nursery. Nurseries have been know to make mistakes...I've gotten a couple of plants that didn't turn out to be what they were supposed to be.