CO2 is good for photosynthesis but high temperatures are bad for photosynthesis. The end result depends on the particular species of plant and regional climate details.
Does "bad for photosynthesis" mean less green, or more green? Warm non dry places aren't usually short on green.
Any place with enough liquid water will tend to be pretty green regardless but it means that the plant will be able to chemically store less energy from sunlight and plants won't grow as quickly.
The more you warm things up the faster water evaporates and so the demand for water goes up. A place that has enough water to be green now may not have enough water to be green if temps go up by very much.