These colors are highly visible when hummingbirds fly, as their colors constantly vary in their complex dance with light. But if you watch a perched hummingbird, like this one, it can tighten its muscles to change its colors even while sitting. Also, if you move slowly or tilt your head as you watch, the colors transform through a rainbow of hues.
I spent quite a while moving at a glacial pace to avoid alarming the hummingbird, taking this photo when it appeared most dramatically gaudy. Walking slowly as you watch a resting hummingbird is similar to seeing the shifting shades of oil floating on water, at least in the sense of underlying physics. As a fan of birds, I would rather watch a hummingbird than an oily puddle any day.