It's because, at character creation or dual-class time, you chose an elemental sphere for the character. Each sphere is a different color, though it can sometimes be hard to tell (though Water is clearly blue, I believe).

If a dual-class Ranger/Cleric chooses a different sphere, the two class levels will be colored differently. (Note that you actually get full access to both spheres, which is a bit of an exploit, but I consider it fair due to how limited a Cleric's selection of high level (4+) spells is.)