It sounds like the success to your method is coming from your curtains at night more than anything else. Heavy curtains that prevent a cooling "heat engine" from setting up on the glass surface is critical to retaining heat at night.
I agree with Xringer--unless you are transmitting heat or visible light back out of the window, what energy comes in effectively stays in, regardless of the color of the objects in the room.
This is why you want to increase your absorber/window area during the day with heat absorbing panels of some kind piped into the room during the day, with the same path blocked off during the night.
Incidentally, this out-transmission of energy is pretty much the largest heat loss in solar heating panels. Most of it occurs in the form of radiant heat loss right back out the glass/lens of the panel.
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