Explanation:
Hοmοzygous means both letters are uppercase, or both are lowercase.
Heterozygous means one is uppercase and the other is lower case.
Dominant is represented by uppercase. Recessive is represented by lowercase.
"Hοmοzygous dominant" is NN.
"Heterozygous" is Nn.
"Hοmοzygous recessive" is nn.
Shade NN red, and shade Nn green. Leave nn unshaded.