Quick answer:

Most city fire hydrant parking laws state that you must be parked at least 15 feet away from a fire hydrant (about a full standard sedan car length). The color of both the curb, and the hydrant itself is irrelevant.

More details:

You’re cruising around in the mid-summer heat, and think you just stumbled onto the perfect parking spot. Instead, you pull up a little closer and see that smug little R2D2-looking bastard smiling right at you. 15 feet is the minimum distance you can park away from a fire hydrant in most municipal codes – which may seem excessively far, especially when every inch of curb in a dense city is so valuable. But there’s a good reason for this fire hydrant parking law. Safety.