Unfortunately with pigment/melanin under the arms, it results from shaving the armpits or using an anti-perspirant deodorant/roll-on which can cause sensitivity to the area. This then results in pigment/melanin/darkness in the area. The knees and the inner thighs can be due to any other reason. This is something that you cannot remove but maybe lighten. What I suggest you do is go see your doctor and he/she might recommend something for you. Let me know what your doctor suggests, but remember what your doctor does recommend will be very strong so remember to wear a sun protection at all times.