2part question
"what would happen to a plant with green and yellow alleles if the two colors are codominant"
" how could a i breed all plants that would look like that? "

Co dominance can be described as a phenomenon which occurs when one allele of a gene is not fully dominant over the recessive allele. As a result, the traits carried by both the alleles can be seen in the organism. A plant having green and yellow co-dominant alleles will show both the colours in it.

Crossing a homozygous green plant with a homozygous yellow plant will result in all the offsprings being co-dominant. The offsprings will have both green and yellow colour in them.