As with a lot of poetry, these lines can have many different meanings, depending on the reader, and what they bring to the reading of the poem from their own personal background. In this poem, the writer describes how a philosophical question from college years ago-do you save the old lady or the painting in a museum fire?- later comes back to her as an old lady, and she finally finds a conclusive answer to it, because she is experiencing it herself. So, as she stands before the painting, it is moving and incredible to her. She describes how
"The colors /within this frame are darker than autumn,/darker even than winter—the browns of earth,/ though earth’s most radiant elements burn/ through the canvas. I know now that woman/ and painting and season are almost one/and all beyond saving by children."
These lines could symbolize how the painting itself is like old age; the colors are darker than autumn or winter. Autumn and winter symbolize the dying of the earth's life that was so abundant in spring and summer. This is like her-she is old and in the "autumn" or "winter" of her life. So, she connects to the dark colors of the painting because they feel similar to the closing of her own life. Then, the next lines could mean that in real life, the earth is the most "radiant" element because it gives so much life, it supplies nutrients and sustenance for most of life on earth, in the painting, seems even more intense. It "burns" through the canvas. This could be the speaker indicating that the painting feels real, alive, important, and worth saving. It too gives life and sustenance, and she feels a connection with it. She emphasizes this meaning in the next lines where she says that she feels one with the painting and with the seasons, and all are vibrant, united, and the same. You cannot separate them when trying which to decide; it's all part of life and its cycles, and no one-children included-can alter the circle of life.
I don't know if that helped at all; this explanation is just one of many possibilities, but hopefully it can get your gears turning a bit. I included links to other possible interpretations. Good luck!