I've had a similar idea in mind for Kharne for a while now. To summarise:
- if you take damage, the character's symbol (@ of course) flashes red briefly. How deep a red depends upon how much damage has been taken.
- if you inflict damage, the symbol of the monster that has taken damage flashes red briefly. This is not graduated.
- if a monster attacks you, its symbol changes briefly to a green colour.
- if a monster has a status effect, for example, held, paralysed, sleeping, poisoned, then its ascii character is shaded a different colour.