Shell color in turtles is inherited via one gene (with two possible alleles). Homozygous dominant individuals have dark green shells. Homozygous recessive individuals have light yellow shells. Heterozygotes have light green shells that are an intermediate color between both homozygotes. This is an example of:

Genetic codiminance.

Explanation:

When there is a dominance in a pair of alleles, the heterozygotes will express the phenotype dictated by the dominant gene.

In codominance, neither gene (allele) is dominant or recessive, so the phenotypes of both alleles will express, neither allele can mask the expression of the other allele. So, the phenotype will be a mixture of both phenotypes, in this case, an intermediary color.