Unlike effects that care only whether or not you spent snow mana to cast a spell, Boreal Outrider cares about the color of that snow mana. It doesn’t matter which part of the total cost to cast the creature spell the snow mana was spent on, only that the spent snow mana matches one of the creature spell’s colors. For example, if a red creature spell has an additional cost to cast it and you spend red mana from a snow source to pay that cost, Boreal Outrider’s ability will trigger.