Let's say that, hypothetically, our sun uses up all of it's hydrogen tonight, and tomorrow morning is the first day of its red giant phase. How long would it take for the star to turn red? How long before we start noticing adverse climate conditions and runaway heating of the Earth?
For a second question, let's say that using magical future technology, we are able to widen the Earth's orbit bit by bit, to keep it inside the goldilocks zone. How would the sky look on this new red-giant-orbiting Earth. How would plants begin to evolve and change color in order to adapt?