Let’s have a look at month by month breakdown;
The Altiplano is nice green and fresh and full of flowering fields, the temperatures are pleasant and nights are relatively warm however the mountains are shrouded in clouds and it often rains. The wetlands are flooded and you will probably see very little animal life. The majority of the roads might be difficult to negotiate due to the rains. Temperatures in the lowlands are high and so is the humidity.
At the Altiplano everything is still green, however there are fewer flowers. The probability of rain is diminishing. Probability of seeing the mountains without cloud cover is much higher. The temperatures and humidity in the lowlands are more bearable, there is less water in the rivers and the wetlands, but the animal life is still fairly dispersed. Majority of the main roads are transitable.
This is the best time to visit Andes and the Altiplano. Temperatures are low, but not extremely low, and the countryside still has some freshness about it, there are still some green colours. Mountains are covered with white snow which has not been stained yet by dust. Rain is very improbable. Lowlands are also cooler, but there is still too much water in the rivers and the wetlands. All the roads are transitable now, except for the little used obscure tracks.