I know one can get paint mixed to match, but if you find an off the shelf color that does already match any of the colors I'd be interested to know the details as that's likely a cheaper and easier paint to get a hold of :)
I almost pursued this on my own when I bought Hepco & Becker's new design of Flash 30 panniers with the color panel inserts, as I thought it might be nice to match them to the green on the bike, but when I fitted the brushed aluminum panels they offer those were a close enough match to the Centenario gas tank that I stuck with them and liked that match better than my mocked up digital image of the panniers with matching green panels.
For the helmet, I would be cautious and do your due diligence on how and if it can safely be painted. I would think that some paints might compromise the shell of the helmet, similar to how cleaning a clear helmet visor with certain products can lead to spiderweb cracks in the plastic.