rainbow coloured aluminum
A quick google search later…
It’s probably steel, though. And there is some wood, but let’s consider the wood a non functional decoration in this case. 
Are you sure that fits in the 18"^3 size restrictions?
No, but in all seriousness, are you asking VEX to make colored metal, or if it’s legal to color on our own? Because it totally is, if you look in the game manual.
I guess that VEX should provide the service when teams are ordering metal structures. More like, you need to pay few extra dollars to have your parts painted in your color choice. That would be so cool! Although i would still choose not to pay for color to save money. 
It would help if you specified exactly what you mean by “colored metal.” Product requests that are specific are much easier to satisfy than vague ones.
I will say, though, that I believe it is infeasible (in a business sense) for VEX to offer different colors of their metal parts. This is because demand for each color would be relatively small, yet the cost of sourcing separate suppliers/processes/orders for each color would be high (due to low demand).
It is also VRC-legal to color your own metal, so teams can already do this for competition if they so choose.
//Andrew
I think they could powdercoat metal to order for a very high rate. This could be for teams that have no access to powdercoating, but you can always just paint it.