In summer, should I close curtains during the day? In summer, that is when it's warmer outside the house than inside, one want to prevent the air in the house to warm up too much. Let's consider that all the windows are kept closed, that they are double panes and made of "Low-E" glass. During the day, does closing the curtains on the windows reduce the heating of the house? If so, does the color of the curtains have a significant impact? 
Intuitively, I would think that once the sunlight goes through the window, it's too late to prevent the greenhouse effect. However, the curtains could create a layer of warm air that would improve the isolation, but maybe the convection would prevent this layer to form.
 A: Windows are transparent to visible light, but opaque to infrared.  If your curtains are dark, they will absorb the visible and emit infrared.  This will cause your house to warm as the light doesn't escape so easily.  If they are lighter, they will be more reflective to visible light and will bounce it back through the window.  This is a rather gross simplification of the process, but this is the basic idea.
A: The heat from direct sunlight is about 1000 watts per meter squared in hot countries. If sunlight passes the glass, the percentage not reflected will heat the room .

During the day, does closing the curtains on the windows reduce the heating of the house?

Yes , depending on the color of the curtain , its reflectance/albedo.

If so, does the color of the curtains have a significant impact? 

Yes. Black curtains will have very low albedo, this means all the energy coming in will be absorbed by the curtain and transformed to infrared radiation, a heat source at the window.
Here is a link for reflectance/albedo of various materials.
White curtains would be close to the reflectance of" white paper sheet - 0.6-0.7".
The 30% absorbed will add to the heat of the room. In hot countries it is wise to have shutters, shadowing the window, which also reduce  heat entering the house even if there is no direct sunlight. Of course if the glass is double glaze or particularly reflective the improvement will be smaller, but there will always be improvement in using external shutters, particularly of white colors.
