In COLORS: Lost Memories the initial actor level is 1 and this gets decreased in the intro by 1, so is effectively 0, RPG_RT allows that. We cap this to 1 in Game_Actor::SetLevel().
This can be seen when looking at savegames created by RPG_RT and Player for this game.
We might have similar problems with attributes such as Atk, Def, Agi, etc.
The number shown in the savegame scene is wrong in RPGRT (for easyrpg saves) in general. The Deep8 dev told me that all values are shown as 1 and when loading e.g. a OFF save in RPGRT the SP are 1 in battle.
Upon further investigation I do not know what this game is doing.
I created a testcase in 2k and 2k3, decreased the level and HP as this game does and both do not allow 0.
However, in this game it is possible (even when I replace the RPG_RT).

Your sanity check branch yields:
Warning: Actor 1: Invalid level 0
Debug: Actor 2: Removing invalid item 18 (of type 0) from equipment slot 1 (needs type 1)
Debug: Actor 3: Removing invalid item 18 (of type 0) from equipment slot 1 (needs type 1)
Debug: Actor 4: Removing invalid item 18 (of type 0) from equipment slot 1 (needs type 1)
Warning: Actor 5: Invalid level 0
Warning: Actor 7: Invalid level 0
This is caused by a manual database edit. The initial_level is set to 0.
Seems that RPG_RT accepts any level for this field and uses stats of 0 and EXP of 0 when the level is invalid.
Adding EXP works as expected (when you have 1 EXP you become LV1) and Level ups, too. But when the new level is below 1 showing the "level up message" will hang RPG_RT. Without the message everything works fine.
Supporting Level 0 and doing a silent sanity log through Output::Debug should be feasible.
Are you sure about the "Level 1 to 0 decrease"? Even when I set the initial level to 0 I can't decrease again to this level with RPG_RT.
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This is caused by a manual database edit. The initial_level is set to 0.
Seems that RPG_RT accepts any level for this field and uses stats of 0 and EXP of 0 when the level is invalid.
Adding EXP works as expected (when you have 1 EXP you become LV1) and Level ups, too. But when the new level is below 1 showing the "level up message" will hang RPG_RT. Without the message everything works fine.
Supporting Level 0 and doing a silent sanity log through Output::Debug should be feasible.