Hard to tell from your photos but your lower lid skin appears a bit thin and crepey. Thin skin can appear darker because the deeper tissues and blood vessels can show through. You can apply over the counter topical vitamin C and alpha hydroxy acid preparations over the lower lid skin daily for a few months to see if you notice any improvement. Be careful and do not get any of it in your eyes. Best thing would be to have an in person evaluation by an oculoplastics (eyelid) specialist to get the most accurate assessment and treatment options. Good luck!
