The phrase "yellow balloons" refers to the color yellow and an object called balloons. It could mean a bunch of balloons that are all yellow in color. Full definition
Stop Yellow Balloon Alerts The Hassle: When I send anything to my networked printer, Windows XP pops up a yellow confirmation bubble telling me the item printed. (entrepreneur.com)
A trail of red and yellow balloons leads guests down a spiral staircase into the basement of a suburban Toronto hotel. (canadianbusiness.com)
Ten days later, Jerry lay choking to death in an oversized yellow balloon that the nurse called «an oxygen tent.» (firstthings.com)