I understand how codominance works. We have two (or more) alleles for one gene, neither having dominance over the other. They are both expressed independently, which means that different phenotypes are visible simultaneously. However, I'm not sure how it really works when we take, say, coat color. If both alleles are active, assuming each one produces a different pigment for each color, that means there are both pigments in a cell. What I see explained is that, if you look closely, each hair has a different color, and when you see it from far away, it gives the impression that the coat is made of one single color. But how can one hair have the same color all throughout? How can the different pigments be aggregated like this? As in, how is it that they do aggregate like this?
The terms dominance/recessivity, additivity, co-dominance, incomplete dominance, etc... express a relationship between genotypes and phenotypes but the mechanism behind this relationship is not being defined by these terms.
There might be a large number of mechanisms associating genotypes at a given locus with phenotypes. There is, in general, no reason to expect a mechanism of relationship of dominance at a given locus to be similar to that at another locus. In other words, there is no general mechanism leading to dominance/recessivity, additivity, co-dominance, incomplete dominance, etc...
On top of that, the actual mechanism in specific cases leading to particular genotypes - phenotype association are often unknown and I don't think we have good knowledge about what mechanisms could be more common or even how to categorize such mechanisms.
You might want to have a look at the posts
or some papers such as
In general, it appears suggested that one needs to know the specifics of the biological pathway to be able to predict dominance relationships and gene interaction network (see Llaurens et al. (2009)).