Modal auxiliary verbs like "could", "would", "should", "might", etc., always appear with a main verb to create a verb phrase. The main verb is the one that carries the action, and it determines whether the sentence is active or passive.
Example:
* Active: She could *eat* a whole pizza. (The main verb "eat" is active.)
* Passive: The pizza could be eaten by her. (The main verb "eaten" is passive.)
Therefore, you can't say "could" is active or passive on its own. It depends on the main verb it's used with.