The poem tells of sadness, loss, seeking maternal comfort. You are very observant to have noticed its masterly use of colors both in parallel and in contrast to each other.
From the first stanza we immediately know something is wrong. "She", presumably a young girl, corresponding to "mothering", is depicted to be running. And that running is directional: she is running home. I read the two occurrences of "blackness" in the first stanza as a symbol of home. Home is mothering and comforting. It is a breeding source of perpetual comfort. That is why it is "mothering". It is symbolized by "blackness" because the girl is black and her mother is black. Black is beautiful and blackness is her source of loving care, a harbor she can always go back to and hide in. That is why it is "mothering". But it is also smothering, because that's the reason "she" ran away from home. She didn't want to stay home so she left. But now she is coming back, running back to it looking for shelter.
white tears icicle gold plains of her face
This line is an epitome of modern poetic creativity, because it forms a golden line. Angelou verbs "icicle" in a very creative and subtle way that makes the line an uncommon English sentence. White tears stream down, dry, and streak her gold-plain-looking face just like icicles. The tears are white because of the sharp contrast they present to her face. Her face is gold colored probably because she is of a lighter skin tone, corroborated by this later line from the poem:
rime of alien dreams befrosts her rich brown face
But as I mentioned, that line is a golden line with two mirroring noun phrases punctuated by a verb. "White tears" corresponds to "gold plains" and their relationship is determined by the verb "icicle". So "gold" here also metaphorically and self-referentially alludes to that.
In terms of "how" the colors are used, they are used creatively. The face of a light skinned Black girl is described as gold plains streaked by white tears. Gold is not a color commonly used to describe skin color. But gold is beautiful, so is a light shade of Black.
Note that the blackness in the first stanza could also be interpreted as the darkness of the night. The night is mothering and smothering because it is anthropomorphized and repressive. It doesn't make too big a difference to my preferred interpretation. Only in that reading the girl comes home in the second stanza as opposed to the first.