bella swan is shown naturally pale... and its clear that she knows not many people are that pale... except the vampires offcourse...
i do believe that most people get tanned in sun... just that some are very prone to sunburns and others dont get tanned easily...
and it all is related to skin structure...
d safest bet is the sunblock as always....
Everyone's skin is different, some people tan easily, some people are naturally pale and don't really tan in the sun. Another reason why some people are pale is iron deficiency. I think it mentions that Bella is just naturally pale