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.
> Clouds within a mile or so of Earth’s surface tend to cool more than they warm. These low, thicker clouds mostly reflect the Sun’s heat. This cools Earth’s surface.
> Clouds high up in the atmosphere have the opposite effect: They tend to warm Earth more than they cool. High, thin clouds trap some of the Sun’s heat. This warms Earth’s surface.
Water vapor accounts for 70% of the greenhouse effect. It's actually the most potent greenhouse gas in the Earth's atmosphere. Increasing a mere 5% is a really, really big deal.
How much light clouds reflect depends on the particulate pollution is in the air, there’s an interesting phenomenon that doesn’t get talked about very often called Global dimming. https://en.m.wikipedia.org/wiki/Global_dimming
Not an expert, but my understanding is, yes, clouds cool the earth during the day, compared to no clouds, and they warm it at night. But on net water vapor is a greenhouse gas: it holds onto heat, and it absorbs the wavelengths emitted by the earth disproportionately more than those emitted by the sun, so it keeps more heat in than out.
Cloud cover reflects infrared, which is why it causes warming.