I think, they are relative to each other. Like, dark eye color is dominant. Other gene might have been expressing its own color too, but the dark color is what we see effectively at the end.

Different genes may have different mechanisms, but the idea is how a single gene can't be enough for making that function effective.

For example, we might have three eye colors which, one is dominant to other, and the last one is recessive to both. Like, A > B > C or something. If gene A somehow would get disappear from the genetic pool, then we might start calling B simply a dominant gene, instead of comparing it with another.