The yellow ribbons that the tanks and soldiers are wearing signify loyalty to the King (as do all of the yellow shirts that people are wearing).
This year is the King's sixtieth year on the throne, making him the longest currently ruling monarch. Everybody owns at least one yellow shirt with the royal crest on it (I have one in the wardrobe upstairs) and have been wearing it on the appropriate occasions. All year it has been the de-facto office uniform.
There were reports of troops approaching the city who were wearing blue ribbons and it was worried that these troops might be loyal to Thaksin. As it turns out they were just wearing a different colour — light blue is associated with the Queen.