Japanese does have words for “white jewel,” “yellow jewel,” and so on, but these refer to gems, not onions!
白玉 (shiratama) white + jewel
This means “white gem,” “pearl,” or “rice-flour dumpling”!
紅玉 (kōgyoku: ruby) red + jewel
You may be wondering why the first character isn’t 赤 (aka(i): red). I don’t know, but 紅 originally meant “red thread” and now simply means “red” or “crimson.”
黄玉 (ōgyoku or kōgyoku: topaz) yellow + jewel
I didn’t know what color a topaz was until I encountered this compound in my kanji dictionary and recognized 黄 as “yellow.” Just one of many instances in which kanji has taught me about English! Note that 黄玉 can have the same reading (kōgyoku) as 紅玉, “ruby.” I imagine that could get a little confusing!
緑玉 (ryokugyoku: emerald) green + jewel
Ryokugyoku is one of those great Japanese words with internal rhymes. Imagine if Dorothy had spoken of the Ryokugyoku-shi again and again, instead of the Emerald City! The Wizard of Oz might have become a comedy just by virtue of that.
青玉 (seigyoku: sapphire) blue + jewel
This is my birthstone, in case you’re wondering what to buy me this September! The same compound (青玉) can also mean “bamboo.”