I read something about this recently. Clouds indeed reflect sunlight, but they also trap IR radiation from the surface that otherwise would make it out in to space. The clouds also radiate IR into space, but the higher the clouds, the cooler they are, and the less they radiate. Apparently, for very high clouds, the net effect is heating (i.e. the heat trapping effect is larger than light reflecting effect), while for low clouds, the net effect is cooling.