I'm writing a story where one of my main characters has a phobia of a color that is associated with highly negative things in their society. First, is this plausible, and second, how would their reactions differ from someone who simply is unnerved by the color due to its negative connotation?
Yes, it’s plausible - phobias can be of anything, and the fact that it’s already associated with bad things in said society makes it more likely (verses any other random color).
Generally, the difference between a phobia and disliking something is the extent and intensity of your character’s reaction. Someone who dislikes the color might say “Ew,” but move on with their life. A specific phobia would more likely involve your character fleeing the color, or even going to great lengths to avoid the color in the first place.
Another difference is that being exposed to that color can result in your character having panic attacks, which produce physical symptoms (see here for some examples of what that could look like).
There is no exact cut off point between disliking something and having a phobia - it’s on a spectrum. For it to qualify as a phobia, however, your character needs to be suffering from serious distress and/or their life needs to be negatively impacted by their fear.
Hope that helps!