The fragility of an ice bridge accounts for the flavor of its one-time use, similar to the way Weatherlight's Gemstone Mine captures the flavor of a mine running dry of precious stones. But what accounts for the Ice Bridge's five-color mana capabilities? The answer is in Rob Alexander's art:
The sun glints off the ice bridge, producing a subtle glow of prismatic light. We can see that this fragile catwalk contains a hint of frozen magic -- hence the potential for rainbow mana.