There is a natural ageing process with leather, which adds character and results in subtle variations to the original colour. General wear and ultra-violet light from natural sunlight also play their part in changing the colour of upholstered furniture.
 
Central heating is the arch enemy of leather furniture, causing drying out of the natural oils and moisture, which results in hardening of the leather and cracks appearing in the finish.
                            What to look for?
                                           Use a magnifying glass and some adhesive tape.
                                                              (Click picture to enlarge)

(1) Test a good area of your suite, such as the back, or outside arm panel – press the tape down firmly, then remove and view through the magnifying glass. It should be free from any matter.

(2) Repeat the test on a worn area, such as the top surface of the arm or a worn seat cushion panel. If there are particles on the tape, it may be the TOP FINISH or worse, the SURFACE COAT. In any event, your furniture needs attention.

(3) A more obvious problem and easily visible with the naked eye, occurs at the head area and on the arms of furniture. It appears as discoloration and hardening of the leather. This is a serious problem, usually associated with OILS, from the skin or hair, which is normally acidic in nature and will eventually rot the leather. The TOP FINISH has broken down and oil has penetrated the leather. This can occasionally be saved, if caught in time, but frequently requires replacement panels.

(4) CUTS and NICKS can often be repaired so that they are not obvious to the naked eye. However, cracks on a seat cushion panel caused by drying out of the leather will normally require a replacement panel which can be colour matched to the present colour of the suite.

                    (5) BROKEN SPRINGS and WORN OUT SEAT PADS both of these can be either
                    repaired or replaced, as appropriate.

                    (6) WORN ANTIQUING  is one of the most common problems. Antique finish is a
                    secondary, sprayed, colour application which is semi-durable. Manufacturers
                     frequently apply insufficient stabilising 'FINISH' coat. Hence, when even a small spillage
                     is wiped away, a startling appearance of the bright under-colour can occur, which only
                     gets worse with more rubbing.

Pictures below show 'before' and 'after' antiquing restoration
(Click to enlarge)
                                               
Before                                                                                        After