If we were to put a highly reflective substance into Venus's upper atmosphere, could we cool it down? I am envisioning adding a highly reflective fine dust or gas, possibly lighter than "air" to the atmosphere. I believe to have the proper effect, it should also reflect infrared light, which causes heat. Apparently Venus already has a high albedo of .75 (reflects 75% of light): http://www.universetoday.com/36833/albedo-of-venus/ . However, if we were able to raise that higher, even up to 99%, would that cool it down, perhaps even making it habitable (temperature wise), or possibly even making it ice cold?
Interesting information about Venus: Venus is hotter than Mercury, despite being nearly twice as far from the Sun. Earth, despite being further from the Sun, receives more energy from the Sun than Venus, due to Venus's very high albedo.
As you might guess by this information, the major factor that keeps Venus hot isn't how much energy it receives from the Sun, but rather its inability to rid itself of energy. While adding a cloud of some high albedo substance to the atmosphere may effectively prevent the Sun from heating Venus, it would also likely prevent infrared radiation from leaving Venus, thus keeping it warm.
As you know, Venus already has a very high albedo. It's thick atmosphere prevents energy from leaving the planet, which is what caused the temperature to rise far above the Earth-like temperatures we believe it once enjoyed. This is caused by IR radiation being emitted from the surface and being reflected back down towards the planet by the atmosphere more detail, which plays an essential part in the greenhouse effect.
In my opinion, a more effectual method in terms of making it habitable would be to find a way to drastically lower the albedo of Venus to allow all the excess energy to escape.
If you can add a material which can reflect optical light (high optical albedo) and transmit IR light (i.e. it does not absorb or reflect too much in IR), it will reduce further heating of Venus and it will gradually cool down.
You cannot lower the IR albedo (by using an IR absorbing material, for example) of Venus since it already has a thick atmosphere with greenhouse gases and if the atmosphere absorbs more IR, it will heat up. You cannot increase IR albedo because it will reflect emitted energy back to Venus and it will still heat up. You need to have a layer which transmits energy and does not reflect or absorb. The transmission of the atmosphere is already low and it cannot be increased, but if you can reduce the amount of incoming light by increasing the optical albedo, and not reduce the transmission at IR wavelengths, Venus will cool down gradually.