"No, actually the greenhouse effect would raise the temperature during the day regardless of whether the sun is at full power or not."
That's completely true, especially if the shelter is set up low enough to keep the draft out. Even a tarp pitched in an A-frame, if it's pitched low, would build up a bit of a temperature differential before convection got the air moving. With the sun veiled by clouds or near the horizon, it will just be slower.
I'd guess that a reflective coating would reduce that effect, by letting less of the heat through the fabric.
Venus, however, is evidence that you don't have to let a lot of heat through the fabric in order to warm it up. All you have to do is keep a small amount more than you lose, since it has an insanely high albedo, and ends up absorbing a lot less energy into its atmosphere than Earth does, yet it's a tad warmer ;)
Then again, this is one of the selling points behind tarps. You don't have to pitch it that low when it's hot, in which case even a slight breeze will keep the interior cool(er) by constantly recycling the air inside, and thereby circumventing the greenhouse effect.