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, as well as 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 try to 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 might see people wearing these colours on the appropriate days.
In Europe, there are still colour codes for certain days and occasions: at funerals people wear grey or black. At weddings the bride wears white - a custom in fact only started when Queen Victoria of Britain did so in the 19th century – but in some cultures (female) wedding guests are not supposed to wear white, black, or scarlet.
European countries also have their own national colours, sometimes related to their monarchy – e.g. the national colour of Holland’s is orange because their royal family is the House of Orange – and people will wear this colour when it’s their national day or at big international sports matches.
But these symbolic colours are always worn in relation to events; they are not related to the days of the week.