Turning up at the opticians and choosing a pair of frames for your glasses, or a simple set of contact lenses is a relatively straight forward process. However, now that more and more people are trying out colored eye contacts, there is a few other considerations to take into account.
Shading for example, is a major factor, especially for those with darker eyes and similar skin tone. If enhancement tinted lenses were to be used in these cases, then the effect would be to potentially darken the appearance too much. Softer shades in an opaque tinted version, would possibly be a better option.
Conversely, if somebody has a lighter complexion and paler eyes, then enhancement lenses would really help the eyes stand out. Eyewear manufacturers can basically custom-make a set of colored lenses to be as under-stated, or as vibrant as is required.