To get the result of having 50% of recessive phenotype (green seeds), it is necessary to have a homozygous recessive parent (yy) crossed with a heterozygous parent (Yy) in order to achieve the 50% Yy (yellow, dominant) and 50% (yy, green, recessive).
The Punnett square below illustrates the above result:
y y
Y Yy Yy
y yy yy
So we get 50% Yy(heterogous, yellow dominant) and 50% yy (homozygous recessive, green).