That doesn't make sense G 1/4" is a thread, you buy tubing in OD/ID such as 16/10mm or 5/8" ID 3/8" OD in imperial units. You need to buy the correct size fittings for the tube or the other way around.
If your after matte black then I would advise you take a look at EK-ZMT and I think you'll find that is exactly what you are after.
JR