What do these numbers across the skills mean

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?

AA stands for auto-attack. Because you don't have to click once every time your character is ready to attack, it is so called auto-attack.

However, there is actually two type of AA, Active and Passive in MOBAs.

Active: You selected a target and your character attacks the target.

Passive: Character that is idle will look for the closest target within attack range, then reach and autoattack (note- there is no passive AA in Smite).

In other MOBAs, passive AA is on, but can be turn off in settings. Another fun fact is I found that more DotA 2 players prefer turning it off, while more LoL players prefer leaving it on.