Just because you've had quirky, colorful nails for a while doesn't mean that bluish, brown, yellow or two-toned darkening is biologically normal for you. What you've come to expect as the regular appearance of your nails may actually be a subtle indication that you have a medical problem that needs attention. In fact, doctors will often inspect a patient's nails for symptoms of disease. It's true that a recent change in the appearance of your nails is a potential warning sign you should discuss with your doctor, but darkening that's been present for a long time can still be significant.
If you have a doctor's appointment coming up, avoid applying polish to your nails for a while, and ask your doctor to have a look at your naked nails. We don't want to alarm you here. Nail coloration (shape and density) can suggest illness, but one symptom, like nail color, can occur across a number of conditions, some minor and others major. If you think your nails may be trying to tell you something, listen to your digits and get a professional assessment.