I wouldn't go by the black marking at all for sexing. If you want to know the sexing of them then do that by venting.
Most yellows stem back to the couple of lines mentioned. There are the odd fresh ones from the lake coming through. I fresh line of lion coves hit the country here, which I got my hands on. The white ones you see in previous page are just the result of years of line breeding. I placed photos of whites ones in this article written by Andrea Watts in Sydney...
http://www.sydneycichlid.com/electric-yellow.htm
North America have huge problems with hybrids, I have seen pics of some, very hard to tell the difference from real yellows. I have seen some Red zeb x yellows for sale in a lfs here, he is passing them on as Tropheops, yet they were imported as orange peacocks.
So back to the topic sorry & pics. I will place some up tomorrow, currently on dial up, take me forever.
Frenchy