Otherwise, the custom and convention in your dojo is what you should pay attention to. Often, the color choice has no significance whatsoever and is simply a matter of personal preference. White hakama are sometimes used in various arts or for certain occasions. Your hakama is in for a lot of wear and tear, especially if you train hard and often. Overall, you should look at several key areas.
Flimsy fabric has a short, sad life and, incidentally, makes for maddeningly wimpy himo.
Go for sturdy, tightly woven fabric you can depend on to last, and check shrinkage and care instructions. The way the hakama is put together is just as important as the fabric. Be sure both the front and back himo are sufficiently long to wrap securely around your waist and hips. And if you are an aikido student, be sure you don't need to tie a knot in the back, where you might roll on it. Look for comfort and fit, too, because these are safety factors.