Yes, they do exist and one of the few colors that I can actually say are rare. I owned several back in late 1998 and 1999 when I owned a loaded mushroom reef. I haven't seen one since then. What is often called yellow today, is nothing more than a muted pale orange polyp. I actually owned 3 half dollar size ricordea which were between a banana yellow and mustard yellow. I parted with them when I got into zoanthids. The yellow ricordea were highly suceptible to color shifting though. Where ever you place them, leave them. Good luck.
Mucho Reef