Phantom, I had the same issue when building my 737. I had no idea. I ended up using Tamyia XF66 light grey which I mixed with XF2 white until I got it light enough. I think it was about 50/50. I used X11 Chrome silver for the leading edge and X32 Titanium silver for the centre of the top of the wings. Looking at pics of the 777 the underside of the fuselage looks to be the same grey as the engine cowlings. The same with the top side of the wings, but with chrome leading edges. Id use XF66 and keep making it lighter until you get the shade you want. Hope thats helpful. I had to eye ball it.
The color is a Boeing grey, the closest colour I had used was Canadian voodoo grey ( FS16515) by model master which is as close to perfect. tamiya xf-80 is another color that is close according to some sources.
The 777 and 767 wings engines and lower fuselage were the same grey(or so close it isn’t funny). The leading edges ( the deicing edges) are more of a Matt aluminium. Let me know if you need any more help, I used to refuel the birds at auckland