A common complaint when we'd show up over a fire in a bright yellow airplane was that the air attack or leadplane couldn't see us. Eventually a national paint scheme was instituted requiring all aircraft to be white with red trim. I'd have certainly bet that a white airplane in a very smoky environment would be tough to spot, but the contrast between the colors makes it visible against nearly any backdrop. If the white disappears, the red stands out, and surprisingly,the white stands out very well, too.
As for composite airplanes, both the heat absorbed by the material, and the UV reflectivity are both issues. The structural strength and integrity of a fiberglass or epoxy surface suffers under high heat, and while you can certainly paint an airplane any color you like, white with a silver underlayer is often the best choice for protecting the structure.
It's also a very easy basic color to paint production airframes that any type of trim can be placed over...meaning paint all the airplanes white and then they can be customized any way you like.
On the ground most of the fire vehicles I've operated, or on which I've operated, have been red. Some agencies, such as the BLM, have gone to light green, which shows up very well, even against dense vegetation. I've seen some white vehicles down there, and from the air they stand out well. However, since there are so many white vehicles on the road, they don't stand out nearly so well. Especially to other drivers; a unique color that separates them from other traffic makes them more conspicuous, which enhances safety and alerts other drivers to their presence. In flight, unless I'm doing something discreet, I want to be seen. I turn on all the lights I've got, I prefer to be painted the most glaringly visible color that can be seen under most conditions, and strangely, white with some contrasting trim works about as well as anything.
Here in the desert, we have yellow school busses. Today is a relatively cool 106 degrees F, and it gets toasty inside vehicles (to say nothing of aircraft structures. Yellow school busses have white tops to reflect more heat and keep them cooler, and most aircraft are painted white for protection from heat, and for visibility. White works.