The error is the black overprint on the right stamp as opposed to the ‘normal’ red overprint. 1906 1c black & purple, two examples, one with error overprint in black, tied by blue Brunei 31 Jan cds on small piece.
The stamp with overprint error is Brunei’s SG11a and has a catalog value of £3,000. According to Stanley Gibbons, only one sheet of the 1¢ received the black overprint. Sold for US$1,400.