Is this a recent change you have noticed? If so she needs to see Opthalmologist to check it out and make sure all is okay.
Is this a new development? Have their been changes in her diet or environment? New pets? Is she complaining of pain? If so, is it a sharp pain, or a dull ache? What makes it worse (bending over, standing up, eating, etc)? Is there anything that makes it better? When you say darker, do you mean color or just brightness? Is it red and itchy? Is the surrounding tissue swollen? Is she complaining or making comments about it being harder to see?
Without more detail I couldn't even hazard a guess... but if you're really worried a trip to the doctor is never a waste for a concerned parent.