How can individual strands of coat hair have only a single color when considering codominance?
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


Why are some genes dominant over others? What is the mechanism behind it?
Evolution of dominance


or some papers such as


Llaurens et al. (2009)


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)).