Rather than trying to match paint colors, couldn't you just take the casing from a Turbo Tap and swap it out with the casing from a Duo/PC Engine Tap?
Nope, the jacks are a different size and it's not possible to get all five of them to line up with the plastic housing.
You could swap cables to create a Reverse Turbo Tap, but then you'd have to use Turbo Pads which wouldn't match the Duo....so you'd have to put the guts of five Turbo Pads into the shells of five Duo Pads - then you'd have five "Duo Pads" with TG-sized plugs going into a black Turbo Tap which fits the Duo. *whew*
Unfortunately, the controllers could no longer be connected directly to the system and there are some one-player games that malfunction when a Tap is detected.