Pink shirts instantly became this season's must-have item, and crowds are mobbing the shops that sell them. In public appearances since then Bhumibol has sported a range of pastel sport coats, changing his style after decades in dark suits and setting new trends with each outing.
As the country gears up to celebrate Bhumibol's 80th birthday Wednesday with fireworks and fanfare, many Thais are honoring their beloved monarch by dressing like him.
"Now the T-shirt craze is green," The Nation newspaper said in a front-page headline two weeks ago after the king stepped out in a celadon colored blazer. Days later, when Bhumibol opted for baby blue, the paper forecast "a run on blue shirts."
Bhumibol, the world's longest serving monarch, enjoys a reverence seldom seen in modern times. Regarded by some as semi-divine, he is revered for his lifelong dedication to helping the country's needy. Bhumibol is a constitutional king with no formal political role but has repeatedly brought calm in times of turbulence and is considered the country's moral authority.
Thais hang his portrait in their homes and shops. At movie theaters, people rise before each film as a slideshow of Bhumibol is played to the king's anthem. Dressing in his honor is nothing new.
Across the country, Thais have donned bright yellow shirts every Monday for the past two years to celebrate Bhumibol's 60th anniversary on the throne in 2006.
Yellow in Buddhist tradition represents Monday, the day of the week Bhumibol was born. It has become a national Monday dress code for civil servants and the daily color of choice for politicians and TV newscasters. Countless other Thais voluntarily wear yellow to publicly express adoration for the king.
Pink is making inroads for a more pressing reason.
Royal astrologers determined earlier this year that pink was an auspicious color for the king's health. On that advice, a pink royal crest was designed for Bhumibol's 80th birthday.
Pink shirts bearing the crest hit markets earlier this year but sales took off after Bhumibol himself wore the color to leave a Bangkok hospital Nov. 7, after three weeks of treatment for heart and other ailments.
Pastel shirts are being sold around the country, but the most desirable come from Phufa, a shop owned by Crown Princess Maha Chakri Sirindhorn.
On a recent morning, hundreds of Thais were lined up outside one of Phufa's Bangkok branches waiting for its doors to open. First in line was Rose Tarin, a 56-year-old civil servant who had been camped out since 4 a.m.
"Wearing pink brings the king luck. I don't want him to be sick," she said, pulling a pink handkerchief from her purse to wipe away tears. "I love the king. I want him to stay with the Thai people forever."
The shop has sold 40,000 pink shirts this month and another 20,000 are being produced ahead of the king's birthday, said a Phufa spokeswoman, Katevalee Napasab.
"We had no idea they would be this popular," she said. "Our factory is making these shirts every day, every minute - day and night."
Bhumibol, who rarely speaks in public, has not reacted to the phenomenon or explained his sudden penchant for pastels. The royal palace says it does not comment on sartorial matters.
"You should ask all the people in Thailand who are wearing the shirts," said Preecha Songkittisuntorn, director of the king's personal affairs division. "It's an expression from the heart."
Mystical and astrological beliefs are entrenched in Thai culture, and some people believe each color carries special meaning. Others say the king is trying to cheer up his subjects.
"Thai people are so worried about the king," said Suchada Kiranandana, the president of Bangkok's prestigious Chulalongkorn University who is also affiliated with Phufa. "When he came out of the hospital in a bright, warm color, the people were so happy. I think the king is trying to please the people."
The king's tailor, Sompop Louilarpprasert, doesn't see what the big fuss is about.
He says the king has worn colorful blazers inside the palace for years, even if he opted for grays and navy blue in public.
At his Bangkok shop a few days before Wednesday's celebrations, Sompop proudly displayed the fabric that he said was requested for the king's birthday suit - a light gray wool with faint pink pinstripes imported from Italy .
"This will be very elegant for his birthday," the king's tailor said. "Much better than a pink jacket."