The different colours represents what you can expect inside:
Each chest have a default colour, that can be overruled through different means. Priority: Blue > Red > Green / Brown (Depending on which is placed in RareItemId)
“AreaChangeTreasureFlag”: true,
By setting this to true in DropRateMaster file, the chest turns blue, and refresh each time you exit and re-enter the area.
“CoinType”: “EDropCoin::D10”,
Enabling coins makes the chest red. This even works if coin ratio is 0, so that it doesn't actually drop any coins.
“RareItemId”: “None”,
“CommonItemId”: “None”,
“RareIngredientId”: “None”,
“CommonIngredientId”: “None”,
If any of these contains a Gear or Weapon item, the chest turns green.
“ShardId”: “None”,
Note that this entry doesn't change colours, even if you do put a gear item there. Even if it has gear in the later slots.
“RareItemId”: “None”,
Setting this to an item like potion will make the chest brown. Unless otherwise over-written.