Answer: This is due to the theory of independent assortment of gene.
This means that gene separate and assort with each other, independently. There a gene that is recessive in the parents may not be recessive in the offspring.
A recessive gene is the gene that makes up the physical appearance of the offspring. While a dominant gene is the gene that exist in the offspring, but is not among the gene that makes up the physical appearance of the offspring.
For the two pure stock parent plants, their have yellow and blue as the dominant gene, while green and white are the recessive gene.
That's is why the yellow and blue gene has a tendency of occurrence in future generations.
For their first progeny which appears to be all green, is because the green gene has sorted themselves independently to be recessive, while others become dominant.
For the second progeny, the recessive gene from the parent plants which was carried by the offspring as dominant gene, now became recessive in some plants.
