It all depends
It all depends on if you are tying for fun or tying for fishing. If they are for fishing, the next question of course would have to be salmon or steelhead. While the color lines are very similar in modern patterns, steelhead colors tend to be on the brighter edge of the spectrum.
To be a "purist" you can't tie Syd Glasso's flies with out yellow. Red is a popular color for steelhead and salmon. Blue is also an increasingly popular color. I would stay away from the orange in the whitting necks, it is really more of a "light salmon", just my personal preference, it is the only die scheme in the necks that I don't like. Claret is also an excellent color for steelhead. One I would definately get is purple....perhaps the best steelhead color in my opionion
I sit down every year and try to determine what I want to tie for when or for fun. Then decide the colors I need. If you only want to do a few flies needing yellow, there are some great heron substitues (dyed Blue eared pheasant for example) and you can pick up 10 or so feathers for less than half the cost of the neck.
If you are tying for fun, pick your favorite colors and have at it....speys are awesome! The Whiting necks are becoming more popular and are now widely regarded as a great subsititue for heron or the more expensive dyed b.e. pheasant.
One other tip I would give the starting spey tyer is to use "cheep" feather like goose shoulder when starting to set the wings. The wings are always the hardest thing to get right when starting. There are three different methods for wing setting, all of them work...try them all and you will find one that works best for you.
Another note: most patterns use bronze mallard flank for the wing. This is one area where you don't want to skimp on price. You can buy a bag of then for a couple of bucks and they will be useless. You have to have prime, MATCHED feathers for a good wing. Just like tying dries, you cant tie a good fly with a bad feather!
PM me if you want more info......speys are great!