Most plant leaves contain yellow and orange carotenoids as well as green chlorophylls. Why then are most leaves greenish

Because the leaves contain a greater concentration of green chlorophylls than yellow and orange carotenoids.

Explanation:

Chlorophyll and carotenoids are both pigments found in the cells of organisms like plants. They have differing color range depending on which wavelength of light they absorb and which they reflect. For example, chlorophyll pigment are green because they reflect green light and absorb others.

According to this question, the leaves of most plants contain yellow and orange carotenoids in addition to green chlorophylls but leaves are mostly green. This is because there is an abundant of chlorophyll pigment than any other pigment in the leaves of most plants. Hence, GREEN COLOR conferred by chlorophyll dominates and masks the color appearance of the other accessory pigments like yellow and orange carotenoids.

"do a testcross with a homozygous recessive plant."