The loose shirt look is one of the most deceivingly easy but in reality really hard to master looks. The point to this look is to look casual and effortless, like you are out for relaxing stroll. According to various style magazines (from Teen Vogue to Elle,) you're supposed to buy a shirt "a few sizes too big or borrow your boyfriend's".
This instruction sounds simple enough, but honestly, who's going to deliberately buy a shirt that doesn't even fit herself? (Unless you're very desperate to achieve this look or you have an amazingly large clothing budget.) And does anyone really walk around with only a few buttons buttoned up? (such as the Tommy Hilfiger look on the right.) Yes, in the Paul Smith photos (on the left), the model actually looks like she's taking a relaxing stroll in the city, but let's face it, most of us wouldn't be able achieve the same effect.
In my very limited experience of buying and wearing white shirts (for the rare formal job related events,) I always feel very... office-like. White shirts are essential for the office-look -like in the Jil Sander photo above, but ultimately, hard to translate into casualwear.