Symbolism, Imagery, Allegory
If you're so inclined, you might want to analyze the fact that D'Artagnan rides a black horse to chase Fouquet, who is mounted on a white horse. Might it signal Fouquet's innocence and purity and D'Artagnan's lack of these qualities? It seems like a bit of an obvious symbol to us, and we think Dumas may have selected those colors simply for their aesthetic qualities, but if you want to find some evidence for it signaling a good guy/bad guy dichotomy, have away at it.