Autumn colours

When we were kids and went out guisin at Halloween we used to say this poem when folk opened the door. Our kids still say it too, although they may be a bit big to go out guisin this year, before being invited to perform a turn and be rewarded with some goodies.

The sky is blue
The grass is green
May we have
our Halloween?

I always used to think it didn't make that much sense in Autumn as blue and green aren't really colours I thought of, but it made sense today looking out of the window.