The seedling forms do seem to vary in how much and how quickly they turn greenish. I work part-time at a nursery - some of the purple-leaves ones that we had went green very soon, and some are still very purple. Since they are growing in the same medium, next to one another, and fed and watered the same, I can only assume it is inherent. If you can get a grafted form - if such are out there - from a very persistant purple-leaf one, you might be OK, but...

I saw, some years ago, so can't now remember where, a very dark leaved smokebush, quite large and elegant, on a tour of some historical house and grounds. I THINK it was at Frederick Church's mansion in the Hudson Valley, but, it was 12 years or more ago...., so my memory fails me. I can remember being impressed by it, however.

Get a named cultivar and the leaves will stay purple.

I usually post over on the GA Gardener forum. I have a Cotinus "Grace" whose leaves have also turned green. They were intensely purple when I bought it this spring. I have it planted in full sun.

Someone else on the GA forum has a lorapetalum that has lost its purple leaf color. She was directed to the following older thread, which suggests some other possible causes (intense heat, low Mg, too much N or P).

Here is a link that might be useful: green leaves

Poster,
Lacyvail is right.
You probably bought your smoke bush at a big box store like home depot or lowes.
The true "Royal Purple" smoke bush will not turn green.
Some smoke bushes are grafted, most do not grow true from seed.
You must order one from a true nursery or visit a specialty nursery to get a good Royal purple smoke bush.
There are also many different purple smoke bushes who have different characteristics.
"Grace" is a hybrid cross between the green smoke bush and Velvet cloak.(which is a gorgeous true purple smoke bush)
"Grace" has blue green leaves with red tinges around the leaves because she is always growing. Gorgeous smoke bush by the way, but she grows very fast and very big.
Research the internet on the different cultivars, there are new cultivars this year you might be interested in.
They are definately worth every dime you pay for them if you buy from a specialty nursery who deals in quality.
Good LucK!

To clarify, 'Grace' is a hybrid between Cotinus obovatus, the native smoke TREE, and a Cotinus coggygria,the ones we call smoke bushes. It will become a small tree.