How do you know...?

[soy.lor.n]

Co-Domiant= Partially expressed phenotype when only one allele present. Full expression when both copies are present. (pastel as an example is only partially expressed, and a super is the full expression of the pastel gene)

But yellowbellies are often called het ivories
maybe it's just because it's a (MUCH) less obvious mutation than the other het co-doms