I'm a bit confused by this, but let me give it a try:
You have yellow pea plants. But pea plants could be yellow or green.
You cross 2 yellow pea plants you get green and yellow.
First, you have to know that yellow is dominant to green in a pea plant.
So, to think about this genetically, give the traits a letter: Y (yellow), y (green)
Knowing that you get a green pea plant out of a cross of two yellow pea plants says that the original yellow pea plants must be heterozygous so Yy
Do the cross of Yy x Yy
Punnett Square:
Y y
Y YY Yy
y Yy yy
By Genotype, you have 1 YY, 2 Yy and 1 yy
By Phenotype you have 3 yellow and 1 green
So, I think your answer is that the parents were heterozgous for pea color because there are only 2 options for pea color (yellow / green). The green pea plant is a homozygous recessive offspring (yy).