atmosphere - Would a high albedo reflective substance cool down Venus?

Eventually, yes.

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.