Why is the colour of a leaf kept in the dark frequently becomes yellow , or pale green? Which pigment do you think is more stable?

In darkness chlorophyll synthesis ceases while degradation continues, leading to rapid loss of the green colour as chlorophyll molecules break down

Chlorophyll a degrades fastest, followed by chlorophyll b, so the leaf’s green intensity fades and the yellow-green of residual chlorophyll b becomes apparent

Accessory pigments—xanthophylls and carotenoids—remain intact longer, so their yellow and orange hues predominate in a dark-kept leaf

Among all pigments carotenoids (including xanthophylls) are most chemically stable under both light and dark conditions, preventing photo-oxidative damage and persisting when chlorophylls degrade

Therefore the pale green or yellow appearance reflects loss of chlorophyll a (and then b) while stable carotenoid pigments persist beneath