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.