If the finish you want can be painted then it will probably be cheaper to add pigments. This will be far the cheapest method unless you are from a low wage area.
If you require a "Chrome Plated" finish then you will have to, er,... chrome plate them. This gets v. expensive as few platers of plastics exist (only 2 in the UK), the fallout rate from the plating is around 20 percent, and then there is the transport back and forward.
Metallic finishes in plastics are produced by adding aluminium flakes - size and geometry of your choice. Weld lines may a problem - the finer the flake the worse it looks. Base polymer is important. Works well in transparents and transucents, (e.g. PC/PS, and PP et.al.)not very good in opaques (e.g. ABS/Nylons/anything filled, etc.)
Suppliers of raw flake include Eckart (Germany)and Silberline (UK), both have worldwide distribution. Any good masterbatch supplier will be able to give you what you require.
Cheers
Harry