You might try the Extend method like so:

use Parse::RecDescent;

my @colors = ('periwinkle','indigo');

my $grammar = q[
    order: quantity color item
    quantity: /\d+/
    color:
    item: 'pen' | 'pencil'
];

my $parser = new Parse::RecDescent ($grammar);
$parser->Extend("color: '" . join(q['|'],@colors) . "'" );
[download]

Or cram it in the grammar...

my $grammar = q[
    order: quantity color item
    quantity: /\d+/
    color: '] . join(q['|'],@colors) . q['
    item: 'pen' | 'pencil'
];
[download]

Update: revdiablo beat me to it... note to self: type less.