The speaker seems to be enchanted with nature, and wants to represent that feeling of curiosity and awe. The whole poem is an act of observation, of fascination with this moment seen in the natural world. Not only does the color of the scene have a natural basis, nature imagery is what sets the speaker's mind on its own journey of contemplation.
Line 3: "Emerald" invokes a particular shade of green, one that is found in nature. Beyond this, "emerald" is also the name for certain kinds of hummingbirds.
Line 4: "Cochineal" is a really hard word that many people won't know. If you're a nature lover (or if you work in fashion), you may pick up on one or both of its meanings—a crimson color and a flying insect from which the crimson dye derives. By using this kind of language, the poem suggests that nature's specificity is incredible—there are numerous shades of red that naturally occur, and the speaker has this precise shade in mind. It's interesting to note how the hummingbird, with its coloring, is really reflecting other elements (jewels and insects) of the natural world. It's as if the bird allows the speaker to really dive into nature and all its elements.
Line 5: Suddenly, the speaker identifies something! She personifies the flowers by saying that they "[adjust]" their heads. Is the bird crashing into the bush or sweeping past it during flight? Or could it be that the flowers are just as amazed by the hummingbird as the speaker is? In that case, it's as though the hummingbird brings the flowers to life. This adds to the speaker's awe about the hummingbird, suggesting that this is such a magical creature that it can make even the flowers seem surprised.