Question

1) A blue alien and a green alien mate and produce 1,000 offspring that are all blue. Another alien couple who share the same genotyope and phenotype as the first couple also mate and produce 1,000 offspring, all of which are blue. Males and females from the two matings then mate with one another. Matings between the blue offspring produce 75% blue and 25% green offspring.

Answer 1

If you're trying to figure out why that happened, it may be because the two aliens  that mated with each other both had a  recessive gene, and since they both carried that gene, some of their babies were born green.

I hope I helped answer your question.

Answer 2

The original blue alien was homozygous dominant (BB). The original green alien was homozygous recessive (bb). ALL of their offspring were Blue, but heterozygous (Bb).  The same is true for the 2nd couple, so you mate the offspring (Bb x Bb) and you get the following genotypes:  BB, Bb, and bb. 75% will be blue (either BB or Bb) and 25% will be green (BB). 
Does this help??