HARVEST - Crop Rotation mechanics?

Hey guys.

Crop rotation is stated as followed:

"Harvest crops in your maps contain only tier 1 plants"

"Harvesting Crops in your Maps has a chance to upgrade the tier of plants of different colours".


I am wondering if there is anything more to this mechanic than simply this? Does it matter how many there is of opposite colours? Or is it just a firm chance to upgrade each and every one of plants of different colours?

https://www.youtube.com/watch?v=_4mvDpMp3vs