They are probably iron deposits as said above. When wheels get hot brake dust can get embedded in the wheel usually in the corners. as laquer is generally thinner here.
Try Iron X first as wonder wheels etc will not be very kind if you do have any laquer peel
iron x is quite good to help you clean wheels anyway and only a tenner or so online.
spray and leave and agitate, may take several applications. if no joy then try wonder wheels.