I'm trying to add RAM to my XPS 8700. I have 2x4GB modules (original) in the white slots and tried to add one 4GB module in the slot closest to the CPU and the PC won't boot. It just beeps and I can't even access the BIOS. The manual says that matching modules in the same color slots is recommended. Does "recommended" mean "required"?

Task manager says I have DDR3 1600mhz so I bought one Corsair module of that type. It should be compatible.

Here's the manual: (page 18) https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_xps_desktop/xps-8700_owner%27s%20manual_en-us.pdf

I want to make sure it's not a different issue before I buy a matching module for the other slot.