I had the pics sent to me also. What the classified add calls a 'yellow' Nile is really just a slightly more yellow Nile. Some of these originate from Gatorland in Florida. They are pretty but they are nothing like some of the Niles I have seen(and maybe you as well) in terms of yellow.
I guess you can sell them as whatever you want and they may be more yellowish but they are clearly in the normal range for the color. The ones I have seen are highlighter yellow and for a Nile fan like me it took me back as I had never seen a Nile look like these.
So I queried the farm owner and he told me about the maize. I may try it on a few of mine. Below is a photo of a farm with yellow Niles.

website
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CDieter
'Reason, observation, and experience; the holy trinity of science.'