Custom Editor - SerializedProperty

[Player.cs]

using UnityEngine;

public class Player : MonoBehaviour {

    public int id;
    
    public string playerName;
    public string backStory;    
    public float health;
    public float damage;
    
    public float weapon1Damage, weapon2Damage;

    public string shoesName;
    public int shoesSize;
    public string shoesType;
    

    // Use this for initialization
    void Start () {
        health = 50;        
    }
	
	// Update is called once per frame
	void Update () {
		
	}
}

[PlayerInspector2.cs] - Editor폴더에 존재

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(Player))]
[CanEditMultipleObjects]
public class PlayerInspector2 : Editor {

    Player player;
    SerializedProperty playerName;
    SerializedProperty playerHealth;
    private void OnEnable()
    {
        player = (Player)target;
        playerName = serializedObject.FindProperty("playerName");
        playerHealth = serializedObject.FindProperty("health");
    }

    public override void OnInspectorGUI()
    {
        serializedObject.Update();
        EditorGUILayout.BeginVertical();
        EditorGUILayout.PropertyField(playerName);

        if(playerHealth.floatValue < 20)
        {
            GUI.color = Color.red;
        }

        if (playerHealth.floatValue > 80)
        {
            GUI.color = Color.green;
        }

        EditorGUILayout.PropertyField(playerHealth);

        GUI.color = Color.white;
        EditorGUILayout.EndVertical();
        serializedObject.ApplyModifiedProperties();
    }
}