The poem starts out like the familiar nursery rhyme, "Roses are red, violets are blue," and then swerves. Instead of "blue," we get "violet," which is a small, almost cheeky joke: violets really are violet, so the line is accurate but flat. That flatness sets us up for the shift in the last two lines, where the tone turns tender and a little sad.
The subject is a woman who is homesick for someone she hasn't met yet. Homesickness is normally about a place or person you've lost, so applying it to a future stranger is odd and moving. It suggests a longing with nothing to hold onto, a sense that someone is missing even though she can't name them.
The mood is quiet, wistful, and gently playful. The simple rhyme and short lines make the feeling seem light, but the idea underneath is lonely. The title, "a flawless image," may hint at how perfect and imagined this absent person is, though that is an inference from the text.