中 means "throughout", not "inside", when it's read じゅう as in the thread I linked above. This じゅう can be attached also to on'yomi words. Hiragana じゅう is sometimes used for this meaning in order to make clear the meaning.
e.g.
町中[まちじゅう]
家中[いえじゅう]
The rest, "kun'yomi + ちゅう" and "on'yomi + なか", don't exist, I think.
Just to make sure, しろなか is rare, as I wrote. This reading sounds classical.