A flower's colour, however, isn't a full-proof guide to a good lunch. That's because the colour can change depending on the angle at which sunlight hits its petals. A yellow flower, for example, may look somewhat blue from one angle and red from another. Scientists call this kind of colour change iridescence. 'It's the same  　 that makes a rainbow appear in a soap bubble or on a CD,' says Beverley Glover. She studies plants at the University of Cambridge in England. In 2009, Glover and her colleagues showed that even when petals look  　 , bees can still tell which flowers likely hold food. But she and others noticed something odd about iridescence. It's not quite as flashy in plants as in other life forms, Glover says. The backs of jewel, beetles, or the wings of certain butterflies, for instance, shine and shimmer a lot more. The researchers tested their hypothesis in the lab. They trained a group of bees to associate fake purple flowers with getting more nectar. Then the team tested the bees. They added non-shimmery  　 flowers with purple-blue and purple-red hues to the bees' flight path. The bees passed the test, ignoring flowers that weren't perfectly purple. Second group of bees was trained to drink from fully flashy, 'perfectly iridescent' purple flowers. But when the team added perfectly iridescent flowers in different hues, the insects checked them for nectar, too. A third group of bees, however, had no problem finding the right flowers when the petals had only a little bit of bling. These bees were trained to drink from '  　 iridescent' purple flowers. When the team added imperfectly iridescent flowers in different hues, the bees weren't confused at all. 'They could still clearly identify the purple flowers as the good ones,' Glover says. That means imperfect iridescence is best for bees.