I suspect that the word you want is 'brown'.
An adjectives modifies a noun or pronoun. There are three adjectives in this sentence. "Small" modifies the noun "antelope". "Brown" modifies the noun "fur". And "top" modifies the noun "half".
This is an interesting example. It has what is known as an implied subject. This sentence is an instruction which is addressed to you. If we were to write it out in full, it would say "You should flip the pancake when the edges are brown" (which, incidentally, is true). The subject of that sentence is you. Pancake is the object. And "when the edges are brown" is an adverbial phrase, which modifies the verb flip.
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A prepositional phrase is not defined by its words, but by the word it modifies. This can be either an adjective or adverb phrase. The mark on the turtle's shell is brown - adjective, modifies mark The paint fell on the turtle's shell. - adverb, modifies fell
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