The colours don't matter, the electricity does not care about the colour of the cable so 4 core cy cable has 4 cores as you have stated all of them are separate from the screen. The screen should be earthed at a star point (where all the earths are connected together to avoid any ground loops in the control box) you only need to earth one end of the cable, depending on the size of the motors I use 1mm others use 1.5 mm. Good luck with your build. ..Clive