Extraterrestrial Planetary Sky Color: does it have to be blue?
Not necessarily blue, although pure nitrogen, ozone, methane and hydrogen- cyanide  all appear as various shades of blue (assuming the incoming starlight is white). Nitrogen dioxide would be a deep brownish red. Some organics and chlorine dioxide are yellow, whilst chlorine is yellowish green. There are many other coloured gasses, but they are not likely to form planetary atmospheres. However, the question implies vegetation, and hence Earth-like photosynthesis, in which case it also implies an Earth-like atmosphere, in which case a normally pale-blue sky is inevitable.