Antonyms for the term "coloring up" could include phrases like "fading away," "losing color," or even "discoloring." When something is coloring up, it is becoming brighter and more vibrant, so it's logical to consider the opposite as something that is becoming duller and less vibrant. Additionally, depending on the context of how "coloring up" is being used, alternative antonyms may include "coloring down" or "coloring in." In these cases, antonyms would be something like "lightening up" or "whitening," which would represent the opposite of adding more color or intensity.