Water fountains often propose two taps, one with cold water and one with water at ambient temperature. In my experience the cold one is labeled with a blue color while the ambient one with white color. Why is that and not the other way around ?
For hot/cold combinations this is a standard to avoid confusion, specially if the faucet is in a different language or uses a different arrange:
From Wikipedia Tap (valve)
If separate taps are fitted, it may not be immediately clear which tap is hot and which is cold.  The hot tap generally has a red indicator while the cold tap generally has a blue or green indicator. In the United States, the taps are frequently also labeled with an "H" or "C". Note that in countries with Romance languages, the letters "C" for hot and "F" for cold are used (from French "chaud"/Italian "caldo"/Spanish "caliente" (hot) and French "froid"/Italian "freddo"/Spanish "frio" (cold)). This can create confusion when English speakers visit these countries or vice versa. Mixer taps may have a red-blue stripe or arrows indicating which side will give hot and which cold.
In most countries, there is a standard arrangement of hot/cold taps. For example, in the United States and many other countries, the hot tap is on the left by building code requirements. Many installations exist where this standard has been ignored (called "crossed connections" by plumbers). Mis-assembly of some single-valve mixer taps will exchange hot and cold even if the fixture has been plumbed correctly.
So you have
red=hot, which are commonly accepted cultural references and have really strong affordances.
Now, this is just a guess, but if you add a new variant (ambient temperature), which is neither cold nor hot, this new variant can't be blue or red. Thus, it hast to use a neutral color which in this case is white.