iOS and RGB Coefficients... do they work?

Tuna which version of XCode are you using now?
When I opened a freshly exported xcode project with the new beta XCode asked if I wanted to update the project settings to the recommended ones. That includes it changing the compiler to LLVM which worked fine for me. Remember there are two places where you can change the compiler to LLVM: In the project build settings and in the 'Target' build settings. The GCC compiler was accidentally set in the 'Target' build setting. If you click the compiler setting in the target build setting and click delete it should un-bold the text indicating that it will then inherit the build settings from the project instead (which is set to LLVM).

The issue is resolved by your suggestion to update the TARGET Build Settings, thanks!

And I can confirm that the GETRGB() function is working fine with the Effect, and my grey objects now receive the color effect I desire!