Is yours in more sun? Less?

I worry about it being in a pot in a newfie winter...is it possible the purple part above the graft died...and the roots survived? Then you'd be seeing whatever the rootstock was...

a variety of factors could cause color changes. how much sun does it get, and from which direction is your plant compared to your aunt's?

you said you gave it tree fert spikes and its in a pot, could be too much fert. did your aunt fert her plant?

any significant size differences between your plant and your aunt's?

did any part of your plant die and turn brown? if so, it might be what mjsee suggests.

are the leaves the same as when you first bought the plant? JM's have a wide variety of leaf patterns, such as thick/thin, 5 or more lobes, etc.

i have an Emperor1, it leafs out purple in the spring, turns green in the summer, turns almost black in late summer then slightly red in the fall.

The other possibility is that these are seedling maples (not grafted). If so, there is a great deal of variability with seedlings and mature plants may not look anything alike nor have any resemblance to their parent(s). It is quite possible a purplish leafed seedling grew out of that coloring and is now showing its true, more mature coloring.

Exactly what type of maple is it? A variety of species are available with colored foliage, not just Japanese maples, but the vast majority, if not all, of these will be grafted forms, not seed grown. And their ability to hold or maintain that coloring will differ widely by species and cultivar.