If you stay on the groomed terrain:
narrower waists, a bit more sidecut and flexes optimized for smooth conditions are best. Stiffer versions will generally hold better on hard snow. More sidecut = shorter turns, more speed control, less sidecut = longer turns, higher speeds, still very controllable. These skis like to be put on edge and arced all day. This is great fun. all core constructions are avialable here, but in the US market now, these skis, other than the "cross" and race models, are either underrated or not high performers. but you can find some really great skis in this group that are super refined.
If you like to go everywhere:
A little more girth (giiiiirrrrrrthyyyyy!) under foot is nice, like let's say between 70 and 78mm underfoot. These skis are usually best behaved with less sidecut in the tail, but still with a good shovel to waist difference to help initiate turns super easy. The wider you go, the more soft snow performance orientation you will get. These skis have different construction to make them handle variable snow conditions. all constructions are possible, and manufacturers start to find interesting ways to maintain torsional stiffness of the ski.
If you want to backwards, play on a rail, be a jibber:
get the cheapest twintip you can find because you're gonna wreck it anyway. skis don't mix well with concrete, steel rails, etc.
If you are an expert level skier who wants to go big off trail all the time:
Buy a ski with a waist between 80-90mm and ski it everyday. it will be like the "everywhere" group, but a little stiffer in the tail, a little less sidecut, and ski super well at high speeds and make wicked long radius turns. At this point you start to find all wood core construction and metal reinforcement pretty prevalently.
If you want to ski powder:
go to store. buy the widest ski in stock. find deep powder. whoop with glee.
as for colors, i've been drawn to neons, so i might recommend staying away from those as i'm attracted to them like a mosquito to an insect lamp.
no, really, colors are just there for looks - get the color you prefer, if possible. the garish cosmetics of some skis definitely do turn me away from them.