The speaker compares themself to the buds of March, those early signs of spring that haven't opened yet but are clearly on their way. The colours listed, green, pink and purple, make the growth feel vivid and full of promise. Nothing is stopping the speaker from blossoming except time, which suggests a feeling of readiness and a little impatience.
The last line shifts the tone. "Why must you be like time" turns directly to a "you," who isn't named. The poem doesn't say who this is, but the line suggests that someone, or something, is holding the speaker back in the same slow, unmovable way that time does. Since the author's name is "That Gay Writer," a reader might connect this to the idea of becoming fully oneself, though the poem itself doesn't say so. The mood is hopeful but restless: the speaker feels sure of the bloom to come, yet frustrated at having to wait for it.