It seems like the Jedi of the time have some standard outfits in a variety of styles and colours - the there are some Jedi who have even more individualistic outfits (like Aayla Secura). If Anakin wore dark robes then I suppose that was just his preference, maybe he thought it went with his status as a prominent Jedi in the war. He may also wanted something that was different from Obi-Wan and most other Jedi - eg Yoda and Mace, to make himself stand out. Although 'going a bit dark side' could be an in-universe reason as well. Maybe a darker mental state made him dress darker and not like most of the other Jedi.