Which colour scatter do I use to represent summer?

It would very much depend on what part of the summer you are trying to create.

If it is at the beginning of the season then everything is still lush and spring fresh, but as you move through the season and things start to dry up due to lack of rain the spring colours change to a more dried out look and continues until autumn.

Just look at nature, look into that field, there are many colours, its not just green there are shades of greens, browns and yellows. So, if you use just one colour scatter you will end up with some nice fields with green carpets in them.

Think about the colours for the part of the season you want to re-create and create a landscape of realistic multi-coloured fields.