"The cold water is in the cup."
Translation:Aia ka wai huʻihuʻi ma ke kīʻaha.
I've been wondering as well. From what I can tell anuanu seems to refer more to feeling cold as in the weather where huʻihuʻi is used more for cold drinks, water maybe food as well. wehewehe.org lists both as meaning cold but the examples they give seem to reflect this as well. I hope that an expert can confirm if this is correct or not.
You're on the right track, Alan, but no one has given a definitive answer to this question probably because it's definitely not an easy question and has lots of overlapping uses. The only minor problem with your conclusions is when food is cold (assuming it has been previously heated) the correct word is "maʻalili." Like, "Eat your dinner before it gets cold!"
Now let's try a few more observations.
Anuanu and huʻihuʻi can both relate to physical feelings, and when the two are both possible (which is quite common), anuanu is generally considered "more cold" than huʻihuʻi which might be better translated as "chilly" rather than cold. (Dictionary definitions don't help you here.) So a season, a day, or a place can be described with either word depending on just how cold it makes you feel, but this depends entirely on the perception of the speaker. Note that the phrase "anuanu loa" is quite common when someone wants to say "really cold." Note also that in traditional sources, both words are often used in the same sentence. And also note that anuanu can be used to describe the person who is feeling cold: "Anuanu au!" ("Huʻihuʻi au" sounds kind of strange.)
As for huʻihuʻi, DL is correct here in accepting only this answer; it would sound really weird to use anuanu in this particular sentence. But why? Good question. If I heard someone talking about "wai anuanu" in a cup, my first reaction would be wondering if the water was feeling cold. Auē!
So are there any "rules" to help us make the choice? If it's referring to water (or presumably any other liquid) mainly intended for drinking, always use huʻihuʻi.
If it's somehow related to your physical reaction (such as to weather, seasons, places, etc.), either anuanu or huʻihuʻi will usually work, but you might want to use anuanu as a stronger "cold" than huʻihuʻi.
If it's being used to describe you (or someone else), anuanu is a better choice than huʻihuʻi. Again, "Anuanu au."
Does this help?