I want to know if sapphire medallion can affect multicoloured spells such as Riku of two reflections.
As long as the spell is blue, it doesn't matter what other colors it is. In other words, yes.
{Sapphire Medallion}
{Riku of Two Reflections}
A multicolor spell is all of its colors. So if you a {ruby medallion} out too, the cost reduction for Riku would be {2}.