The unknown type of corn had to be yellow.
There were offspring that were red and yellow.
The red corn only has recessive alleles (r and r) so, the yellow alleles( R) didn't came from this type. They came from a yellow type corn.
And since the offspring were NOT all yellow (the dominant color) we know that the yellow corn was heterozygous for the color trait (Rr).
50% red (rr)
50% yellow (Rr)
There were offspring that were red and yellow.
The red corn only has recessive alleles (r and r) so, the yellow alleles( R) didn't came from this type. They came from a yellow type corn.
And since the offspring were NOT all yellow (the dominant color) we know that the yellow corn was heterozygous for the color trait (Rr).
50% red (rr)
50% yellow (Rr)