The genotypes of the parents are AArr and aaRr
Since, half the offspring trees produce big red apples and half produce big yellow apples we can conclude:
That all of the offspring inherited dominant trait for the size of apple (all of them are big) so, AA from one parent and aa from another can combine and form all heterozygous genotypes Aa. On the other hand, trait for the apple color is expressed 50-50-% (rr and Rr form Rr Rr rr rr genotypes).