An adverb typically answers questions such as "how", "when", "where", "how much", or "to what extent". An adverb modifying an adjective will often answer "how" or "to what extent" for adjectives. Here are examples:

"Sally noticed the brilliantly colored sunset out the break room window."

Brilliantly is the adverb modifying the adjective colored.

"He described the incident as slamming on his brakes when an extremely black dark suddenly appeared out of the darkness."

Extremely is the adverb modifying the adjective black.