Here are a few:

Marvy Metallic:
Scribble on several colors of Marvy Metallic marker
onto the black cs in big, bold patches. Mist with
water until it is runny. Blast with the heat gun to
swirl and mix the colors. Verrrrry mysterious!
Bwuhhahahahaha!

Another:

Mix your favorite interference colors (interference
PearlEx) with a binder (like Future floor finish,
liquid gum Arabic or clear acrylic paint). Sponge or
daub on the colors. Very pretty.

Yet another:

Daub on Stewart Metallic Palette ink with a stipple
brush or sponge. Easy Peasy!

And another:

Use any of the Primary Elements and the solution recommended for glossy
cardstocks.  Sponge or brush on.

Finally:

Credit card streak.  Using an old credit card or other hard plastic
card, apply drops of metallic acrylic paints to the edge.  Scrape the
card along the glossy black cardstock.  Leaves streaky blends of
color.  Looks interesting with the glitter paints, too.