"I wore pink on that day and people got excited," the king said in his annual address to the nation late Tuesday. "I'm old but I don't want to dress so boring."

"If I kept wearing a uniform, it would be boring. I should wear other colours like yellow and pink," he added.

The king later returned to hospital to visit his ailing elder sister Galyani, once wearing a green jacket and then a blue one, sparking daily waves of green and blue shirts on Bangkok's streets.

Pink, however, seems to have captured the nation's fashion sense.

"I intend to wear a pink shirt every day until the end of this year, and next year too, as it gives blessings to the king," said Prapa Suwankul, a 68-year-old Bangkok business owner, as she bought her third pink polo shirt and an extra one for her daughter.

"I want the king to be in good health as long as possible," she told AFP.

Especially popular are pink shirts from Phufa shops, which are owned by the king's daughter, Crown Princess Maha Chakri Sirindhorn, where long lines spring up every time a new shipment arrives.

Phufa's two shops in Bangkok sold about 40,000 pink T-shirts last month and an additional 20,000 are hitting the shelves for the king's birthday.

"The first lot of 10,000 pink shirts was gone in just two hours. Demand from the public is a lot higher than we expected," said Suchart Thongsa, who works at a Bangkok branch of Phufa.

"People start lining up when a new shipment of pink shirts arrives. Some get here at 4:00 or 5:00 am," he told AFP.

The power of pink also crosses the gender divide.

"As a man, I don't mind wearing pink. As a Thai, I will do anything that I can to express my respect to his majesty, because I love and believe in him," said corporate employee Sompote Traiputtitham, who was hunting to find a pink shirt still in stock.

"I have one already but I want another one, so I can wear pink more often because I heard pink is good for his health," the 43-year-old told AFP.

It's not just clothing sales that are benefiting from the national euphoria surrounding the birthday of the man who is widely held up as the living embodiment of the nation's very identity.

Commemorative books and photos document the king's life. CDs and music boxes play music composed by the jazz-loving king. Calendars, cards, wristbands and watches emblazoned with royal seals are all big sellers, shopkeepers say.

Two books honouring the monarch are at the top of the best seller list compiled by the B2S book store in downtown Bangkok.

"They are selling very fast -- especially the books, but any item, including things like CDs and stationary, which represents the king is the subject of enormous public demand," said sales manager Pankhwan Ongbutr told AFP.