Sometimes foreigner might wonder how the colors of King or Queen decided.
The colors of King and Queen are according to the their birthdays. In Thai tradition, there is an astrological rule that assigns color for each day of the week.

The color is assigned based on the color of the God who protects the day. For example, the God of Sunday is Surya which has red color. These colors of the day are the traditional Thai birthday colors. As following this, since the King Bhumibol was born on Monday, on his birthday Thailand will be decorated with yellow color.

Following is the table of color of the day.

Sunday - Red
Monday - Yellow
Tuesday - Pink
Wednesday - Green
Thursday - Orange
Friday - Blue
Saturday - Purple