Huh. I don't know about trends and Pepper X's association with the mustard color, but I think mustard peppers are pretty cool. The Chocolate Bhutlah is supposed to be significantly hotter than the Mustard Bhutlah, though. I got the Mustard one because I figured it was the least expensive superhot, and so maybe it was easy to grow and get lots of seeds. Well, it did turn out to be pretty easy to grow (much easier than regular Habaneros, anyway). I'm not sure how seedy it is, yet.
Mine don't turn yellow, though. They start yellow (instead of green) and turn green-ish/mustard when ripe, and when extra ripe they get a bit of orange on them (but not all over).
Here's where I got them.
Do you mean Mustard Bhutlahs in general or my Mustard Bhutlah specifically, or all mustard-ripening peppers?
The black ones start green (with black on them) and turn orange (without much, if any, black on them), however. The black ones are never yellow.
Nice pictures!