All Padawans of races that grow hair have a braid behind their right ear. Braids have a yellow band at the top and a red band at the bottom. Additional beads or other color bands are added to mark the Padawan’s area/s of study.
The yellow band is applied when a Master takes on the Padawan before 13 standard years of age. The red band is applied after 16 standard years of age when he/she completes their Temple courses and starts studying independently under the guidance of his or her Master.
Anakin had red and blue bands. Mechanics and piloting.
Obi Wan had red and yellow bands. Piloting and weapons.
Ahsoka in Clone Wars is still young and so has only one yellow bead for her notable saber technique.
Braids are cut by the Master’s lightsaber upon the Padawan’s completion of the Trials (generally around 20 standard years but often earlier or later depending on the student). Jedi who are expelled from the Order prior to completing their training have their braids ripped out rather than cut. Ouch!
Source: Various SWEU books, but mostly “The Jedi Apprentice”