Leg color is determined by 3 to 5 genes. It is not super simple and there can be variations in the color based on those genes.My RIR have excellent leg color but my RB do not. So I fed a high quality feed with marigold for about 8 months to see if there was a difference. I figured since I had birds of all ages over that period of time it would be a good trial period for a couple of groups.
I no longer feed that feed. It was the FRM gold that I know so many people on here like.
I'm sure it's a good feed but it had other ingredients I didn't want and my birds look just as good on my own ration without marigold. And the RB who need more yellow showed no improvement in leg color over that 8 months in any age group.
So... My conclusion... leg color is genetic. Just my opinion.
The study I quoted found that marigold extract increased the yellow in Broiler chickens. They could have a different gene make up.
You will notice the leg color of yellow shanked hens that are good egg layers fading out over the laying season. Like a lot of things it is both genetic and pigment related.
No, you cannot take a white legged chicken and make it have yellow legs by feeding marigold extract. The Genes are not there for that.