My experience is more “feels” than fact I suppose, but I’ve always seen it that any adjective or noun playing adverb to another adjective or participle should be hyphenated to the word it describes.

Red-hot coals (coals that are hot to the point of being red)

Red hot coals (coals that are both hot and red)

Ruby-red shoes (shoes that are as red as rubies)

Ruby red shoes (ruby shoes that are red)

Smooth-talking rogue (a rogue who talks smoothly)

Smooth talking rogue (a smooth rogue who talks)

Bamboo-eating panda (a panda who eats bamboo)

Bamboo eating panda (bamboo is eating a panda)