It is a Thai custom to wear certain colours on certain days of the week. These colours relate to specific planets and thus to the days of week, but also to the monarchy; wearing the colour is a sign of deference and loyalty.
Yellow, blue and pink shirts are especially popular nowadays. These refer to the colours of the royal family: yellow for HM King Bhumibol, born on a Monday; blue for HM Queen Sirikit, born on a Friday; and pink for the now deceased but still revered King Chulalongkorn, born on a Tuesday. On 5 December, the King’s birthday, most people will wear yellow – as well as fly yellow flags. On 12 August, the Queen’s birthday, people and flags form an ocean of blue. In recent times, when the political situation in Thailand has been tense, coloured T-shirts have been worn at demonstrations as a show of loyalty to the monarchy.
You may well see people wearing these colours on the appropriate days.