So you have a dual channel memory setup, purchase some nice Corsair Vengeance RGB Pro memory and then the rest of the DIMM slots is vacant. Corsair came up with a clever yet simple solution to give the PC a better finish.
You could purchase another memory kit running 180 bucks to fill that void, however, to complete the looks you can now purchase a dummy kit to match up the existing DIMM modules. For 40 bucks you can get a CMWLEKIT2W/B kit that matches the looks and style of the DIMMS 100%. The dummies have a fully working RGB LED lighting system that you can sync up on iCUE. Corsair is offering these in black and white SKUs and both
Clever idea.
Corsair offers Vengeance RGB Pro Light Enhancement Kit (Dummy DDR4 Modules)