It makes no difference at all for the temperature under the hood to first order whether heat leaves the water through the hose surface or the radiator. The water is going to the radiator which is in front of the engine. The air passes through the radiator from outside the engine compartment and passes into the engine compartment. The goal is to lower the water temperature to keep the engine from overheating. , both for efficiency and mechanical reasons.
A bad consequence of this is that now the air under the hood has changed density, hence a changing amount of O-2 per cubic foot. In the "old days" of carburetors this necessitated a choke on the carb. The carb was set up to run with hot air, and the choke was used to restrict the flow while the air and intake manifold were cold (and air was dense) then was opened when the air and engine components got warm so that the fuel to air ratio was proper enough to keep the engine running. High performance carbed engines might have "ram air" which meant they always got atmospheric temp air and the carb settings would be radically different. The temperature of the air-fuel mixture and the engine components also affects the efficiency of the engine.
Today, fuel injected engines measure intake air temperature and mass flow to balance the air:fuel ratio via computers. Most fuel-injection engines also have a (poorly designed) cool air (meaning not engine compartment) intake so that there isn't a dramatic temperature change for the intake air during driving. Engine compartment temp and manifold temp does have a small effect, but all in all, the color of water hoses is a zero sum game, thermodynamically. Now aesthetically, some people love chrome, some love black, others want to match the paint color, and most of us don't care.
BTW, my '69 Mustang and my motorcycle have carburetors, so I have starting issues in the winter.
________________________________________
From: Phys-l [phys-l-bounces@www.phys-l.org] on behalf of trappe@physics.utexas.edu [trappe@physics.utexas.edu]
Sent: Tuesday, January 13, 2015 9:51 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] physics and potatoes
...snip....
there is another interesting problem. Should you tape radiator hoses
with foil tape or with black tape? Actually since engine heat from
outside the hose is usually much less than hot water heat inside the
hose, you want black tape (hence black hoses under the hood). Better
make it flat black.