Although the colors the speaker uses are pretty concrete, the actual descriptions in the poem are really vague, and this creates an interesting tension between what is known and what is unknown as we read the poem. On the one hand, the specific shades allow the reader to picture something concrete—we can see the exact colors the speaker wants us to. On the other hand, since the colors are presented seemingly out of context, we're kind of at a loss to attach them to anything. By describing the action this way, the speaker gets at what it's like to see something happen really quickly: a blur of color. We experience the same kind of confusion and vague sensation while reading the poem that the speaker did while seeing the hummingbird!
Lines 3-4: The way the hummingbird is described is pretty important. Rather than just saying, "The hummingbird was red and green," the speaker also describes how she experiences those colors—quickly, but intensely. The phrase "Rush of Cochineal" helps to show how the speaker's sensory impression of the bird comes really quickly, while "Resonance of Emerald" not only gets at the bird's green color but gives it an emotional attachment. That green was worth remembering, for the speaker.
The speaker also uses extremely specific shades of green and red, using words that have interesting connotations. "Emerald" brings to mind not only a particular shade of green, but also a very expensive gemstone. This gives the bird a kind of royal, rare quality. Also, certain hummingbirds are actually called "emeralds," so this may be another hint that the speaker gives us as to the subject of her poem. "Cochineal" is an extremely difficult word, referring to a shade of crimson that is extracted as a dye from a particular kind of flying insect (and just the females at that). Once again, the poem's color imagery leads us back to a natural source for the color (after all, emeralds are naturally-occurring stones). (For more on nature imagery in the poem, we suggest you check out, um…"Nature Imagery.")