1. yellow is the rank of the champ , purple is the rank of its ascension and red is the rank of its awakening

2 Books , great hall , better gear .

3. uncommon are bad , simple . Rare at 6* is another story , depends on the stats but 4* epic and leggo are worst in my opinion than a 6* rare .

Edit: also the champion power level is more or less accurate .. it takes a general model for all champs when certain champ will do better without following that model .. 

So just to add a bit more flavour to the response:

Don't look at champ power. It's not relevant. That said, to improve your champs, focus on the stats they actually care about. DPS champs need CRATE/CDMG/[PrimaryStat]. Debuffers need SPD/ACC. "Tanks" need DEF/HP/RES. Buffers need DEF/HP/SPD. Etc.

To your second question - strictly speaking, higher level artifacts are better than higher rarity ones. However, as Lyryal said, you should avoid spending money upgrading low-quality artifacts. A level 16 epic or legendary item is almost certainly going to be better than an uncommon one. The reason for this is that the higher the rarity, the more substats you start with. And you can't upgrade substats past the rarity of the artifact before it shifts to unlocking new substats instead.

As an example, an uncommon artifact starts with only one substat. That means your first upgrade (at lv4) will enhance that substat, but the next three (at 8/12/16) add new substats. Conversely, an epic one starts with three substats, so the first three upgrades enhance them, while the last one adds a new one.

For the replies. And I have another question. Is it worth it to use the chaos ores to reconfigure artifacts?

Its not worth gathering them but no harm in using them , best is used on a piece of gear that already rolled 2-3x on same stat but isnt the proper main stat , gives it a second chance to be worth keeping . 

Example , plain stat chestpiece , that rolled double plain stat on substats , can use it on that , may roll % mainstats and the double roll could hit speed or another decent stat instead . If that makes any sense .

Think of it has a way to turn a horrible stated piece of gear that double or triple roll when giving it few upgrades(4-12) to become a decent piece of gear but dont hold your breath either , the rng is fully at work so the result may be pointless , but thats the main purpose i found for them personally.

The short answer there is almost certainly no. I highly doubt you have items worth using it on right now.