Of course it's a metaphor. While there is a scientific basis in color temperature as it describes light coming from a comparable heat source, 'warm' and 'cool' are not related to this. It is a psychological association with these colors – blue is viewed as 'cool' by us (or at least in Western cultures), while red/yellow is seen as hot, even though in terms of color temp. a bluish tint comes from a higher temperature. From the Wikipedia page on
color temperature:
Apple should be describing color changes not in terms of "less/more warm", which means nothing to a typical user, but the effect it has on them or the situation in which the setting would be used. Look at the way lightbulbs are marketed – LED is usually marketed as more "revealing" while traditional yellowish bulbs are marketed as "soft" lighting. So something more like "Crisp" and "Soft" would be better, or "Focused" and "Relaxed".
Really though, I think Apple has begun to offer too many options in their software, so I'd rather they get rid of that slider and not allow manual control at all. I'd also re-design the whole settings interface and make it more icon-based.