Why are aircraft coloured in yellow during construction, repair or overhaul?
For unassembled and new planes or planes not yet painted the green/yellow color you see is the anti-corrosive coating on the aluminum 


  ...every unpainted airplane is nominally green from being coated
  (typically) with an anti-corrosive green zinc chromate or zinc
  phosphate primer over the aluminum skins. The different shades of
  green simply tell you that different vendors produced the different
  pieces—they don’t all use the exact same primer.


Originally it was some variation on Zinc Chromate but it was found to be fairly toxic other things have been used over time which lead to the more green color and even some potentially "chrome-free" options.