Buying by color or patterns in never a good idea as Ray said. They will change based on their moods, the water quality, their enviroment and just because they can. Since you mentioned the Ghost, that is a term used by AMI to described some of their H. comes.
Listed below are some of the other terms they use to describe their seahorses. They are all marketing terms use to enhanced the appeal of the seahorses. ( I C & P this from another site I am active on.)
H. comes
White Tiger Tail
Ghost Tiger Tail
Yellow Tiger Tail (I think these are now in the UK market. I read on another site about it.)
H. reidi
Black Seahorse
Yellow Seahorse
Leopard Yellow
Yellow White Band
Bright Yellow
Golden Flaked
White
Lemon
Orange
Leopard Orange
Orange White Band
Red
So if you are in the market for a H. comes or H. reidi, buy them for that fact only and not for a desire color or pattern. Just do a search on here and see how many people are not happy with it or are trying to find a way for their seahorses to return to their original color/pattern.
Their is nothing to know about a standard H. comes VS a Ghost H. comes. The care for them is the same because they are the same kind of seahorse only with a different color at the moment.
Kind Regards,
Tim