Fugitive
Describes a colour that fades or changes quickly, especially in light.
Also: fugitive colour, non-permanent
A fugitive colour does not last. Exposed to light, air or damp, it fades, darkens or shifts hue. Many early synthetic dyes and some plant colours are fugitive. Old watercolours sometimes show the result: pink skies gone grey where a red lake has vanished, or green leaves turned blue because the yellow faded first. Vincent van Gogh used a red that has since faded in several paintings, changing their balance. Paint makers now rate their colours for permanence, and fugitive ones are sold mainly for work meant to be reproduced, not kept.