Could l wire a thermostat outside and and set is to 68 degrees and use it as a switch to prevent the air conditioner condenser from running when below 68 degrees. And use in-line with the yellow wire.
Regardless of how good of an idea this is, what you want is possible and is actually common for heat pump systems (albeit at freezing temperatures). Your outdoor unit just has a pair of wires going to it that allow the indoor thermostat to switch it on. You would just need to find an outdoor thermostat that has the range you want and wire it in to the control wire. Its low voltage, so it's a simple and safe DIY project as long as you disconnect the power to the outside unit before opening the panel.
One issue is that the indoor thermostat would still turn on the blower inside, but it would just circulate air in the house and not cool. The blower could come on for a long time if circulating the air doesn't cool the area the thermostat is in.