Yellow is supposed to be recessive to white.
So if any hens carry the recessive gene for yellow legs, breeding those hens with him will give you some chicks will yellow legs. The other chicks will show white legs but carry the gene for yellow legs.
If any hens have only the genes for white legs, breeding them to the yellow-legged male will give chicks that show white legs but do carry the gene for yellow legs, so those chicks have a chance to produce offspring with yellow legs in later generations.