Under normal circumstances, the problem facing the Apollo spacecraft designers was not how to keep the spacecraft warm, but how to keep it cool. Apollo had a lot of heat sources inside: the astronauts (only a small factor) and scads of electronic equipment, which in those days was far less efficient than today's equivalents. The problem was bad enough that the Lunar Module actually used a coolant fluid that circulated (absorbing heat from the equipment), and was then vented overboard, taking the heat with it.

To minimize the amount of heat that the cooling systems had to handle, the Apollo spacecraft (plural) were made very reflective (think shiny side of the aluminum foil). Therefore there was little thermal gain from solar radiation -- by design.

There was another factor as well, although I don't know whether it made any difference in the net thermal equation. It was not desirable to have one side of the spacecraft baking in the sun (and the other side freezing in shadow) for long periods, so it was standard practice to place the ship in "passive thermal mode". This was a controlled roll, with a rate on the order of one RPM, that allowed the ship to be evenly exposed on all surfaces. This was done during the Apollo 13 mission, even after the explosion. While it may not have reduced the solar heating of the interior, it did prevent then from pointing the windows at the sun and getting some warmth that way.

Indeed, it did get quite cold in the spacecraft, because most of the electronic gear was turned off. IIRC, the cabin temperature dropped to the low 40's (F) -- not much warmer than a refrigerator. And the astronauts didn't have any warm clothing. I'm not sure why the EVA suits couldn't be used, but even if they could, there were only two of them.