I'm considering growing some sushi radishes, but what are they called, because I'm considering growing some this summer after ordering them from a seed catalog.
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.