What are yellow sushi radishes called?
I think you're talking about takuan? I don't know if I'd call it a sushi radish, but it's yellow and Japanese, at least.

It's just pickled daikon, not a naturally yellow radish:


  The finished takuan is usually yellow in color and quite pungent, though most mass-produced takuan uses salt or syrup to reduce the dehydration time, and is colored artificially.


You can have a look on Google image search to confirm it's what you're thinking of.