I would like red spoke nipples also, but am suspicious of aluminium alloy for wheels, I also have heard much about them, light and pretty they may be, but, hey, they are under a lot of tension and tension that varies as the wheel goes around, so me, i kind of don't trust them. So, it will be plated brass then, or stainless steel, if nipples are available in that material.
but as to colouring other materials, well, there are a few chemical plating materials on the market, have a look at what is out there, black should be easy to obtain. Red, h,mm, I wonder, copper plating perhaps or lead oxide.
I will have a look in my college research notes, plenty of info in there on plating and other surface treatments.
oh, I think you can get red dyed titanium.