and color some bushesyellow and some redbut the grief in my heartis stronger than they (13-16)
These colors seem to provide a contrast with the white flowers of the plum tree, and those eerie white flowers at the end of the poem. Whereas the white flowers might suggest death (think deathly-white and bone-white) the brightness of these yellows and reds provides a stark contrast to our speaker's grief. And while red and yellow are pretty bold, strong colors, they're just not bright enough to outshine the speaker's sadness. Nice try, nature.
that in the meadows,at the edge of the heavy woodsin the distance, he sawtrees of white flowers. (21-24)
In her mind these distant trees of white flowers seem to represent a place where she can escape the sorrow and isolation she's experiencing. Or, to put it more bluntly: she associates these white blooms with death. But, interestingly, not in the same way as the white flowers the plum tree in her yard. No, those are all about her husband's death. But these far-off white flowers seem much more closely tied to her own.