Apparently, platinum is the main metal/ingredient/mineral/whatever
that causes hair to be blonde, so the phrase "platinum blonde" is
really true.

Can you confirm this and also provide the main 'metal' that causes the
main other natural hair colours (e.g. black, auburn/brown, dark brown,
strawberry blonde/ginger, red, and - if one exists - grey)?

Hello Davidbodycombe

Hair colour in humans is caused by two different pigments called
eumelanin and phaeomelanin.  Differences in colour are simply down to
the amounts of each pigment in the hair.  The hair fibre itself has no
colour and it is the pigment chemicals that cause the colour.

The colour of the hair is not down to platinum or any other metal
element being present I'm afraid.

Dark coloured (Dark brown, black) hair have more eumelanins whereas
light coloured hair (red or blonde) have more phaeomelanins.  Grey
hair occurs when the cells that produce these pigments have died and
therefore reduced pigment production occurs.

These excellent pages will help explain this in full detail for you:

How do we get all these different hair colors and shades 
http://www.keratin.com/as/as005.shtml

What is hair colour?
http://www.keratin.com/as/as001.shtml

Hair colour chemistry
http://www.keratin.com/as/as002.shtml

Ok, I must have been barking up the wrong tree there. Thanks.

There is one case I know of in which hair color is due to the presence
of a metallic element.  It's not a "natural" color, though.

The active ingredient in "progressive" hair colorants used to cover up
gray hair, such as "Grecian Formula", is actually lead acetate.  The
lead reacts with the sulfur in S-containing amino acids in one's hair
(as well as some sulfur contained in the product itself) to produce
lead sulfide.  Lead sulfide is a dark pigment that darkens gray hair. 
See http://home.nycap.rr.com/useless/grecian_formula/ or
http://www.reachoutmichigan.org/funexperiments/quick/quantlead.html