Karsa - not too sure where you heard that, but it's not exactly true. Yellow labs come from three locations in lake Malawi, Lion's Cove, and Kakusa, and Mbowe Island. At these locations ALL of the Labidochromis caeruleus are yellow - from what I've seen, linebreeding hasn't done anything to change this colour in any way. White Labidochromis do occur near Lion's Cove (a few miles south), but at Lion's Cove itself (and for a couple miles North) all the Lab's are yellow.
Any 'yellow labs' without the black in the fins are cross-bred with something else. I'm not exactly sure what, but as a guess I'd say either Ps. saulosi or Met. estherae (red zebra).