The poet's mention of the colour yellow, besides red, blue, and green, would have lent effectively to the imagery by enriching the visual palette of the garden. In the poem, the garden is compared to a canvas where nature's artwork comes alive through different colours. Adding yellow would enhance this imagery as it is the colour of sunshine and spring flowers like marigolds and sunflowers, which perfectly aligns with the line "Awaiting spring's vibrant hue". Furthermore, the context highlights that the interaction of different colours symbolises diversity and harmony. Including yellow alongside green, red, and blue would make this diversity more complete and vibrant, creating a richer, more vivid picture of nature's artwork in the reader's mind.