Possible Clues:

• A shade inexperienced on the river
• Colour thought of as colour of an African river
• Pale colour of engineer working to contain loch
• Part of raised veneer, gel intensifies colour
• Colour of light initially fixed by engineer — sorted!
• Colour of duck, say, on easternmost edges of Lake Michigan
• English innocent has nothing on in the shade in Africa
• Feverish engineer's left in the shade
• Checking van in lock-up, engineer altered tone
• Student stops engineer working in shade