I would not get into which colour is best suited but I have built a number of koi ponds for both myself and a few friends and in each case I have used G4 pond paint which is a superb product. It is basically a fibreglass resin paint to give a plastic finish and although I no longer keep koi, I still stay in touch with a friend who pond is painted with G4 and is now in it's 8th year, no issues whatsoever.
This is the product
http://www.ebay.co.uk/itm/G4-POND-PAINT-SEALER-CONCRETE-SEAL-WATERPROOF-PAINT-SEALANT-GARDEN-KOI-FISH-BOND-/190645891580
Hope that helps
Phil
