What Causes Dark Spots On Soles Of Feet And Knuckles?

i have dark spots on the soles of my feet and 4 near my knuckles, they are brown, almost black, they are the size of a pen dot , to smaller, they are mostly round but some are irregular shaped, it looks like ink is in my skin. there are over 40 on my right foot, ther are also some that are light brown. And they appeared yesterday and wont wash off. im very light skinned and im only 16. could this be something serious?