I'm often asked "which colours should I buy?" when I'm demonstrating Promarkers, and there's no easy answer here. It really depends on what you use them for. But I can make a few suggestions - my basic advice is to buy, say 3 graduated colours per colour family and build up your collection that way.
Usually I suggest the following as a basic starter kit:
For skin I use: Burnt Orange, Coral, Sunkissed Pink and Blush (but you can leave out the Burnt Orange until you are more confident)
I have picked out a selection of browns that you can use for many things eg hair, trees, clothing, hedgehogs, teddy bears, sepia effects etc: Walnut, Cocoa, Cinnamon, Tan, Almond
Once you have those basics you can choose which direction you want your collection to go, so I've listed the colours I use for various hair colours below.
Light Blonde Hair: Primrose, Pastel Yellow, Sandstone, Caramel
Dark Blonde Hair: Ivory, Buttercup, Mustard, Spice
Red Hair: Chestnut, Terracotta, Spice, Ginger, Oatmeal
Brunette Hair: Black, Walnut, Cocoa, Cinnamon, Tan, Almond
Black Hair: Black, Ice Grey 3 & 5, Indigo Blue, Powder Blue
For a monochrome effect you can use any of the grey collections - each will give you a slighlty different effect, and there are 5 in each family... Cool Grey 1-5, Ice Grey 1-5, Warm Grey 1-5.
**edited to add** Letraset have released 3 new blending 12-packs - they each contain 6 specially selected blending pairs plus a blender pen so are fantastic value for money.
The colours in the 3 packs are:
Muted: Buttercup and Tulip Yellow, Tea Green and Soft Green, Cornflower and Blue Pearl, Orchid and Purple, Dusky Rose and Antique Pink, Caramel and Cinnamon
Pastel: Vanilla and Satin, Meadow Green and Apple, Pastel Blue and Powder Blue, Lavender and Lilac, Pastel Pink and Baby Pink, Putty and Tan
Vivid: Sunflower and Gold, Pear Green and Olive Green, China Blue and Midnight Blue, Plum and Aubergine, Crimson and Burgundy, Burnt Umber and Burnt Sienna
note there are no colour duplications across the 3 sets so it's a great way to build up your collection.