When legumes begin to grow, they depend on the nutrients stored in cotyledons. The newly sprouted buds are yellow because they contain no chlorophyll. The bud tips of plants are stimulated by hormones such as auxin and grow continuously. In the process of re-growth, some cells at the plant bud tips began to differentiate into mesophyll cells, and there were chloroplasts in mesophyll cells, which were divided into chlorophyll a and chlorophyll b, which were blue-green and yellow-green, respectively, so the leaves looked good.

(All the above are personal opinions. )