The blue and white PCIe x16 slots share 16 PCIe lanes.
If you have one graphics card, put it in the blue slot and it will get all 16 lanes.
If you have two graphics cards, put them in the blue and white slots. The blue slot will get eight lanes and the white slot will get eight lanes.
The black PCIe x16 slot only has four PCIe lanes connected to it. You shouldn't use that for a graphics card at all.