The colours of the aurora are determined by the composition of gases in the Earth's atmosphere, the altitude at which the aurora occurs, the density of the atmosphere, and the level of energy involved.
Green, the most common colour seen from the ground, is produced when charged particles collide with oxygen at lower altitudes (around 100-300 km). Occasionally, the lower edge of an aurora will have a pink or crimson fringe, which is produced by nitrogen molecules (around 100 km).
Higher in the atmosphere (300-400 km), collisions with atomic oxygen produce reds instead of greens. Since the atmosphere is less dense at higher altitudes, it takes more energy and more time to produce red light (up to two minutes), whereas green light can be made quickly at lower altitudes (about one second).
Hydrogen and helium can also produce blue and purple, but those colours tend to be difficult for our eyes to see against the night sky.
To see aurora you need clear and dark sky. During very large auroral events, the aurora may be seen throughout the US and Europe, but these events are rare. During an extreme event in 1958, aurora was reported to be seen from Mexico City. During average activity levels, auroral displays will be overhead at high northern or southern latitudes. Places like Whitehorse, Yukon; Yellowknife, NWT; Gillam, Manitoba; the southern tip of Greenland; Reykjavik, Iceland; Tromsø, Norway; and the northern coast of Siberia all offer a good chance to view the aurora overhead. In North Dakota, Michigan, Quebec, and central Scandinavia, you might be able to see aurora on the northern horizon when activity picks up a little. In the southern hemisphere the aurora has to be fairly active before it can be seen from places other than Antarctica. Hobart, Tasmania, and the southern tip of New Zealand have about the same chance of seeing aurora as Vancouver, BC, South Dakota, Michigan, Scotland, or St. Petersburg. Fairly strong auroral activity is required for aurora viewing in those locations. The best time to watch for aurora is around midnight, but aurora occurs throughout the night. There are very few places on Earth where one can see aurora during the day. Since clear sky and darkness are essential to see aurora, the best time is dictated by the weather, and by the sunrise and sunset times. The moon is also very bright, and should be taken into account when deciding on a period to travel for the purpose of auroral observation. You might see aurora from dusk to dawn throughout the night. The chances are higher for the three or four hours around midnight.
No! It is a myth that the northern lights happen only when it's cold. They happen year-round and since there is more darkness to see them in the winter people associate the cold winter with the auroras. August and September is a great time to see the Aurora while wearing a T-shirt and shorts!