Also, variety is seen in the Aussie's eye color, and they are often heterochromatic. An early nickname for the breed was "ghost-eye dog". Aussie eyes may be any shade of brown or blue; they may have two different colored eyes, or even have bicolored or "split eyes" (for example, a half-brown, half-blue eye), which appear to be similar to the merle coloration. Merled eyes exist where one color is mixed in and swirled with another. Any combination of eye color is fine in the breed standard, so long as the eyes are healthy. In general, however, black Aussies (self, bi-color, or tri-color) tend to have brown eyes, while red (self, bi-color, or tri-color) Aussies usually have amber eyes, though these Aussies may also carry the blue-eyed gene.