While the color of the sheath doesn't affect the physical properties of the wire or signal, they are important as visual indicators of function. If the pickup wires were all the same color, there'd be no good, quick way to identify signal vs. ground, as PFDarkside referenced.
In a kit that's not pre-wired, where it's all just a bunch of parts in a box, then certainly, the color of the wire doesn't matter as long as you're consistent in where each wire goes and what it's meant to do. But as best-practice, following a certain convention is recommended under the assumption that you're not going to be the only person looking at the wiring, or that you're going to remember what you did in a few years.
In home wiring, even if you're not the electrician who ran the wire from the breaker box to your fixtures, if you open up the fixture and see white, black, and bare, you'd know which wire does what and where it goes. And be able to safely replace an outlet or switch or install a new light.