Green numbers beside the items in smite

The different color price is basically a separator, if they were all yellow it would seem a little bit odd.

The start of a game is a perfect example to give, in Assault Mode you have enough gold at the start to fully complete an item, would you rather those 3 prices be yellow? Or the colors they are now to signify upgrades?

Edit: This answer is no longer correct, but it's here for historical reasons. Apparently now Mages do Magical damage with their auto attacks as well. See this answer: https://gaming.stackexchange.com/a/126776/1721.

All basic attacks do physical damage. That's what balances Physical and Magical gods. Magical damage gods do a lot of damage with their skills, but their basic attacks are weak. Physical damage gods have strong basic attacks, but weaker skills. (Some of them have strong skills, but those ones have melee autoattacks so they're not overpowered.)

You can tell what type of damage a skill does by looking at its description.

There are two things you have to look at, what type of damage an ability does and what stat it scales with. In the above picture you can see the line Damage: Magical. That means that ability does Magical Damage, which in-turn means an enemies protection against this skill would depend on their Magical Protection stat.

Then look at the Damage per hit line. The numbers in green are the base amount of damage that skill does at each level of its evolution, in this picture, the skill does 100 base damage per hit when it is level 1, and 180 base damage per hit when it is level 5. Then look in the parenthesis next to it and it will tell you that that ability does an addition amount of damage per hit that is equal to 70% of your Magical Power.

The second line, the one in the parenthesis tells you what stat that ability scales with, so if you want that ability to be strong, you have to increase the correct stat.