There’s kind of a flaw with the plan of colonizing Mars “in case of nuclear winter” : Mars is already an irradiated wasteland.
Mars does not have a strong enough magnetosphere to protect living beings against cosmic and stellar ionizing radiation. IF nuclear war breaks out, humans will still have to survive in fallout shelters for centuries, be they on Earth or Mars, except that the people on Earth will actually be able to come out eventually. BECAUSE EARTH WILL STILL HAVE ITS MAGNETOSPHERE, whereas Mars will never have that. The upper atmosphere of Venus is actuallly more suitable for Life than Mars.