thank you for your query.tanning
of the toe knuckles is a common issue.and it is easily resolvable.
Medical cause for it is Anemia
or deficiency of heamoglobin in blood.
it can be easily treated by using iron tablets for 30 days
in case its just due to tanning,you can start with a basic regimen to be followed daily in summer.
1.start by soaking your feet in warm water with salt for 20 mins.
2.scrub the knuckles thoroughly with lemon slice esp on the tanned areas.
3.wash it off and apply moisturizer
these three simple steps will ensure your feet remain soft and tan free..
wishing you good health