What methods can be used to make a dark colored window become opaque

This is not meant to be a complete answer, but might help with your understanding of it. My understanding of the physics of windows is as follows -

Nearly all radiation (visible, UV, infrared, every other type too) that travels from outside the house into the house and is absorbed (not reflected back out again) is converted into heat. I would expect that exceptions (such as the energy from UV light causing a chemical change, such as when it fades your couch, or waves that induce a current in a conductor) are relatively negligible.

This means any radiation that can be reflected from the window to prevent it entering the house reduces the heat load. Also, if the radiation is absorbed into the window glass (eg through tinting it), then some of the heat from the window glass will be lost to the outside (through both conductance/convection and some radiated heat) which is an additional benefit.

A heated window pane would like lose more heat to the outside than the inside because the wind would help keep the temperature difference between the glass and the adjacent outside air larger than the temperature difference between the glass and the inside air.

First, try to prevent the sun from hitting the building in the first place. Shade trees, large overhangs, louvers can all do this. There may be micro louver screens available that appear to be heavy duty insect screens but are actually very small louvers. You can easily see through the louvers, but they prevent the sun from hitting the glass.

Second, reduce the impact of what sun does hit the building by reflecting it away. Transparent reflective film on the window and light colored wall finish will work for this measure.

Third, reduce conducted heat. Insulate the wall so the hot outer surface cannot easily heat the inner surface. Use dual glazing the same way, the hot outer pane cannot easily heat the inner pane.

Fourth, prevent the hot inner surfaces from heating the room's air. Typically drapery or similar treatments. Not just the window, the wall too. Even sheer drapes you can see through are better than nothing. They trap air between the glass and the fabric, creating additional insulating value to keep the heat out of the room, and reduce the transmitted light that would have further heated the room interior.

I've covered how to reduce heat gain through the various modes of how it gets into the room, but I've only scratched the surface of ways to do this. Be creative. Figure out ways to achieve what I've outlined with materials that are readily available to you.