https://zhidao.baidu.com/question/1894099130155161660.html
虹 hóng 在口语中单用时也可以念 jiàng
虹jiàng义同"hóng虹"，见hóng。 限于单用：
如雨过天晴后，天边出~了。
再如，东虹（jiàng）风，西虹（jiàng）雨。读这个音
https://reurl.cc/bmEDZE
When used it alone, you can pronounce it "jiang4"
But personally I have never heard of the pronunciation of "虹 jiàng/jiang4",
I always thought it was only pronounced "虹 hóng/hong2" (I'm not very
knowledgeable and the more common pronunciation of 虹 is "hóng" hahahaha).
Then I looked up the pronunciation and explanation of 虹 on the Chinese web,
yes, there is "jiàng".
According to the explanation, "虹 jiàng" is only used alone, it cannot be
combined with other characters, it also means "rainbow", and it only appears
in colloquial speech.
E.g: Tiānshàng chū jiàng le.
天上出虹了。/天上 出 虹 了。
(there is a rainbow in the sky)