Plant cells contain cell organell called as plastids. There are three types of plastids leucoplasts, chromoplasts and chloroplasts. Leucoplasts are white in clour and genreally they are responsible for storage of starch. Chromoplass are colorfull and they contain pigments like xanthophyll, caroteins etc which give different colours like yellow and orange. Chloroplasts are green in colour and they have major pigment chlorophyll which is green in colour besides the xanthophylls and caroteins. These plastids can interchange their functions as per requirement.
Spinach looks green due to presence of chlorophyll and papaya contain xanthophyll and caroteins, waermelon red due to anthocyanins.