4th column is reserved by green, so you want to settle green at 4th column.
But if there is set of green or something at nexts, for example, set of green purple or of green red, green is linked by I-shape.
This is not good shape. To avoid such a I-shaped link, double green should set on 6th vertical.
Just remenber as "because it is at the beginning". The purpose is secure of linking space.
