I know how you feel - my palomino is the same. Last year his winter coat was really really light, almost white. This year, his winter coat is a lot darker (this is only my second winter with him.) Last year his summer coat shedded out a lot dark than his winter coat. So, I'm hoping this year he will be even darker yet
But I doubt he will. There is a supplement called Gold as Sun, but if your horse is already getting good nutrition, I doubt you would notice much of a difference. And I think you need to feed it before they shed out in the spring. It's been said that the main ingredients are paprika and I think seaweed. Soo, I've been trying the paprika too
Probably won't do anything magical like I'm hoping, but I thought it'd be worth a try too.