If you're going to go with a coat enhancer, I would suggest Black-As-Knight over Gold-As-Sun if you're looking at getting a darker coat. BAK gives a darker tone, whereas GAS enhances gold highlights. Coat Enhancers
But instead of paying extra money for coat enhancers, look at the ingredients. Paprika is one of the main ones, and is what I feed to my palomino mare. They actually feed paprika to flamingos in many zoos to keep their pink extra pinkie for zoo visitors.
With paprika however, you have to be careful to take your horse off of it about 2 weeks before a show if you show at a rated level because it can show up as an anti-inflammatory because of the capsaicin naturally found in the pepper. Also, be sure to keep the paprika in a dry, cool location. Paprika becomes hot/spicy when heated. I feed about 1/2 tablespoon each per feeding to my horses and that does it.
For the tail, I would use some M-T-G.. Works like a miracle. Product Details
You also might consider adding some sort of oil to your horses feed to help with coat and hoof health overall. I use corn oil because it costs less, but flax oil is supposed to be more balanced for horses. I used to feed whole flax seeds, but that got expensive quick too.