I hear that certain koi are not actually marbles but operate around a different set of genes.
What does your koi look like? 1 color, 2, or 3?
What colors does your metallic possess? Black/blond, blue/green/steel, red/orange/yellow? All these things will influence your progeny.
If your koi is just a marble, then I would expect multicolored marbles.
But if your koi is a true koi type, you might get a mostly multicolored (non-marbles) and a few koi (apparently the genes are partial dominant)
But this is all guess work. I would encourage you to find out personally and I'm interested in the outcomes myself--I've never seen anyone attempt this cross, and I would like to incorporate koi patterns into my project, if it is possible.