Ideas on Damage severity

Hey! I am currently making a single-player RPG plugin.

I'm planning to add something like a damage indicator where colors differ depending on the severity of the damage recieved.

I had an idea to do something like
LastDamage (math thingy) with MaxHealth

Which I can't recall doing because my brain is too small to remember things from the past and I need your help to do math things!!!!!

 

PHP:

$severity = $player->getHealth() - $player->getLastDamageCause()->getFinalDamage();

if ($serverity < 3); // Do stuff  if health lower than 3

switch ($serverity) {
    case 3:
        // Do stuff if health equals 3
        break;

    default:
        // Else...
        break;
}