We will meet this situation: the gold color of the socket connector is different from the normal plating gold color, or the different parts are the different color on the same product.
The reason of abnormal color in the gold layer of the ENIG PCB:

The impurities of the plating gold raw material
When adding the chemical material into the gold plating liquid, if the impurities exceed the maximum capacity of the plating liquid, it will affect the gold color and light soon. If it is organic impurities, by the HULL CELL experiment, we will find the gold color will be dark or flowered and the position is uncertain. If it is metal impurities interference then will narrow the valid current density range. The HULL CELL experiment shows what the low current density end of the test piece can’t be plated light or the high current density end is not lighting or can’t be plated. Then we got the flush or nigrescense plating layer, there is the obvious color change in the via.


The higher current density during gold plating
Due to the wrong calculating of the gold plating parts area, the wrong data will > the real surface area, then make the plating current higher, or because of the small amplitude when using the shake gold plating, then the part or the full plating crystal will be crude.

The gold plating liquid aged
If the gold plating liquid remains too long then the impurities will be excessive and cumulative which leads to the abnormal gold color.

The alloy content in the hard gold plating changes
In order to improve the hardness and hard wearing of the socket connectors, we usually use hard gold plating and the often we use the gold cobalt alloy and gold nickel alloy. The plating gold color will change along with the cobalt and gold content in the plating liquid. If the cobalt content in the plating liquid is higher then the gold color will be redder; if the nickel content in the liquid is higher the gold color will become light; if there is the big change of the plating liquid while the different parts of the same product is not in the same plating slot, there will be the different gold color of the same batch PCB product.