A bird's colour is normally a consequence of structural pigment in the feathers & so a red bird looks red because it has red feathers!
However some birds, especially Hummingbirds, have complex feather structures which act as diffraction gratings splitting white light into component parts, some of which is absorbed & some reflected. The variable colours produced by this process are bright, reflective, metallic and ephemeral. Colours produced by this means constantly vary in intensity giving the birds a sparkle which brings the feathers to life. This moment-to moment variation is dependent on the angle & intensity of the incident light. The process is illustrated in this brief slideshow of a feeding male Ruby-Topaz Hummingbird flitting between flowers (taken in Trinidad).