If g is green and g is white what cross will guarantee 50% of the progeny will be green and 50% will be white?

A cross having one partner with heterozygous alleles for green  (Gg), and partner with homozygous white allele (gg), will lead to 50% of the progeny having white color and 50% having the green color. This will happen because 50% of the offsprings will bear a Gg leading to green color due to the presence of dominant G.
Remaining 50% will be white due to gg allele, which was contributed each from both of the parents.