Sunlight is the key to seeing a hummingbird's colors. The brilliant, iridescent colors are caused by the refraction of light by the structures of certain feathers. Like prisms, feather structures split light into its component colors. Only certain frequencies are refracted back to viewers' eyes. The resulting colors are more vivid and iridescent than those of birds with only pigmented feathers. 

"What a beautiful hummingbird with the green iridescence of its feathers shining against the orange zinnia. What a balancing act, hanging on to the Zinnia with its tiny little feet." 
Amy Evoniuk  Newburgh, Indiana  September 23, 2014