The BPMA is currently showing an exhibit titled "Treasures of the Archive." It is part of the London 2010 Festival of Stamps and will remain on display until next spring. There's an online version of the exhibit here.
One of Arnold Machin's plaster casts is included in the display, but of more interest to me was a sheet of Machin color trials. The most interesting of these was the last (fifth) row that showed five metallic colors. The accompanying text says that these metallic trials were "recently discovered." Curator Douglas Muir told me that meant they were discovered after he wrote his 1997 book, A Timeless Classic: The evolution of Machin's icon.
For the record, here are the trials displayed on the page. My color names are not precise, and the lighting was dim, so just take these as giving a general idea of what is shown.
Row 1: Dark colors
Crimson (similar to issued pre-decimal 6d)
Row 2: Dark colors
Row 3: Light gradated colors with dark heads
Row 4: Bicolored
Black with light green(?) head
Black with orange head
Dark blue with pink head (similar to issued 1977 £5)
Black with grey head
Row 5: Metallic
Gold background with black head and value, matte
(there were three of these that appeared identical)
Silver background with lighter silver head and value, shiny
Gold background with light gold head, matte
(Many thanks to Glenn Morgan, who confirmed some of the colors when I couldn't interpret my scrawled notes after I returned home.)