Cold is an adjective in that context.
adverb or adjective
"cold" is an adjective, and "water" is a noun.
Cold is a noun and an adjective. Noun: I have a cold. Adjective: Expect cold weather.
Mostly is an adverb. It is used to describe the frequency, extent, or degree of an action or quality.
So is an adverb in that example.
Digestion is mostly used as a verb. It describes an action.
Colder is an adjective. It's the comparative form of cold.
Bone is mostly used as a noun. There is a crude usage that would be a verb.
The word terror is a noun. It is mostly an uncountable noun.
The word eerily is an adverb. You can easily spot adverbs as they mostly end in the suffix -ly.
It can be an adjective, noun, or a verb. It is mostly an adjective, though.
Transition can be used as a noun and a verb. Noun: e.g. process of change from one style to another. Verb: to make a transition. (This usage is mostly used in the US)