There is no way to say someone "acts white" and for it to be a compliment as long as one is refering to race.
I am african-american. I and my immediate family are light-skinned. This was added into the mix when my school-aged peers told me I was "acting white." I did not "talk black" and (my greatest offense) did well in school and loved learning. For this, I was accused of "acting white". I also had "good hair" (not quite as "nappy" as many people's chemically untreated hair), which meant to my peers that I wasn't really black and must be ignorant of my heritage (and any unshady goings-on in my family). Even when I went to college, the "accusation" wouldn't die. I called my mother at work one day and her co-worker picked-up. My mother later told me that when the woman handed her the phone, she asked my mother if I went to a white college. Apparently, my black parents were incapable of teaching me proper grammar and enunciation, so I must be going to a white college.
The phrase is often used by peers to marginalize minority students who are doing well academically and who do not want to get involved in the less responsible activities engaged in by many youths. It is meant to discourage such behavior and, in my case, was used to attempt to re/assimilate individuals into whatever has currently been termed "black culture" and the "proper/popular" attitude for the race at the moment.
A special was done on television about this phenomenon a few years ago. A number of minority students described the pain and discouragement they dealt with from peers who "accused" them of being "white". One student had a grandmother who didn't want him 'up in her house talkin' all white.' He had to resort to slang and mispronunciations to be acceptable to her when he went to visit.