Who can help me to remember about colors of the aura

0 = White
1~4 = Blue
5 ~ 8 = Green
9 ~ 12 = Golden
13 ~ 14 = Red
15 = Purple

At certain levels, the intensity of the auras change. For instance, the level 14 charge-up aura is slightly bigger and more rapid than the level 13 aura.

it depends on armor or armor+boots+legs?

It depends on how many times the item is upgraded. All items (top, bottom, shoes, gloves, and weapon) go by this. Gloves and weapons, however, do not give any auras. They just glow whatever color they're upgraded to.