Do not level everyone at the same time. Choose only the heroes that you like, or your mains. Putting everyone in the same level is not wise. Equal? Yes. Effective? Nope.


In the equip screen, you will notice a color scheme,

Green: An equipment is available for the hero, or is ready to be crafted with complete recipes.
Yellow: An equipment is available or ready to be crafted but the hero does not hit the required level.
No Color: No available equipment.

Promoting heroes unlocks the new tier of skills. Below are the rankings in the ascending order.

White
Green
Green+1
Blue
Blue +1
Blue +2
Purple
Purple +1
Purple +2
Purple +3
Purple +4
Orange