Quote from their website:
DoubleO Scenics have produced a range of Static Grass blends, containing up to six different colours in order to recreate each of the various shades and textures of our Static Grass and we've named them for their most suitable use or season.
In general the majority of modellers will usually model the summer season by default as they prefer leaves on the trees and flowers in the gardens etc. but there are a growing number of modellers wishing to model other seasons and some wishing to model all four seasons across their layout.
Whichever you choose to model you will notice from the pictures below that the DoubleO Scenics Static Grass blends are not a uniform shade of green like other makes but each blend contains a varying degree of tones for a more realistic looking grass (even the Growing Cereal Crop).