In other words, were Whites allowed to use a segregated Black bathroom or drinking fountain at will without fear of getting in trouble with the law? Could they sit anywhere on the bus, even in the back? Was there some de facto privilege that they wouldn't get prosecuted in the same way an African-American would if he/she violated segregation? Or was the law equally applied to Whites who broke the rules?
In his book The Land Where Blues Began, folklorist Alan Lomax says he was arrested for shaking hands with Robert Johnson's mother in the 1940s. Alan Lomax was white and Robert Johnson (and his mother) were black. There's also an interview of Alan Lomax collected by the Association for Cultural Equity that mentions his arrest: http://research.culturalequity.org/rc-b2/get-dil-details.do?sessionId=84.
I believe Alan Lomax was also threatened with arrest because he called a black man 'Mr.' instead of 'boy' but I cannot find the source for that. Perhaps some good redditor will know more than I do.
Whites were expected to follow the rules of segregation and faced arrest if they did not.