Hornbeam hedging copes better with soils which get wet - Beech hates wet feet. As for Miracle Gro or any fertilizer, it sounds as if there's a problem with some of the plants you have, and fertilizing them isn't necessarily the answer - diagnosing whatever the problem is, then correcting it (if possible) is the way to go.

any chance you can improve the drainage to save you going to the effort/cost of replacing them. There is a natural colour variation between different plants. our hedge has some less green plants and they are mature plants.
feeding around the soil wont hurt as they can be hungry plants but that wont be the cause of the differences you report.

I agree Hornbeam would do better and the leaves are similar..
Our beech hedge comes into leaf rather erratically, not all at the same time
What time of year was or picture taken? Ours looks like that in early autumn as not all the plants go brown at the same time either.

Thanks for the replys. Yes pictures were taken early autumn the part of the hedge that is more yellow is just not doing as well as the rest. I put some chicken manure on it last week and will keep doing it on the bad parts. I also planted 100 hornbeam yesterday so will see how that goes. Hopefully not to late to plant them. I noticed that the soil was like clay where the beech didn't grow. So I hope the hornbeam will have a better chance