Theoritically we have read that ketones do not get directly oxidized by potassium permanganate aqueous solution. However, when I tried doing so with acetone, the pink color did diffuse to red, then orange and then yellow;but very gradually. This means acetone was oxidized by kmno4! How should I account for this?

Acetone is very slowly oxidised by permanganate, but the reaction isn't useful since it produces a number of different products.