All crotons have new green leaves, as they age they aquire variegation. for very good variegation you need part-sun position indoors: south or west is best. otherwise the leaves will remain plain . on yours older leaves are reddish-black, so if you don’t provide bright light the newer leaves will just turn muddy brownish.

it is an outside plant and as such should be grown outdoors in summer. only a few cultivars can develop good variegation indoors in part-sun: ’Petra’ is one if the best.