[MKM] Implement Niv-Mizzet, Guildpact (#11787)

* [MKM] Implement Niv-Mizzet, Guildpact

* Address comments

Mage/src/main/java/mage/abilities/keyword/HexproofFromMulticoloredAbility.java

@@ -0,0 +1,47 @@
+package mage.abilities.keyword;
+
+import java.io.ObjectStreamException;
+
+import mage.MageObject;
+import mage.game.Game;
+
+public class HexproofFromMulticoloredAbility extends HexproofBaseAbility {
+
+    private static final HexproofFromMulticoloredAbility instance;
+
+    static {
+        instance = new HexproofFromMulticoloredAbility();
+    }
+
+    private Object readResolve() throws ObjectStreamException {
+        return instance;
+    }
+
+    public static HexproofFromMulticoloredAbility getInstance() {
+        return instance;
+    }
+
+    private HexproofFromMulticoloredAbility() {
+        super();
+    }
+
+    @Override
+    public HexproofFromMulticoloredAbility copy() {
+        return instance;
+    }
+
+    @Override
+    public boolean checkObject(MageObject source, Game game) {
+        return source.getColor().isMulticolored();
+    }
+
+    @Override
+    public String getRule() {
+        return "hexproof from multicolored <i>(This creature can't be the target of multicolored spells or abilities your opponents control.)</i>";
+    }
+
+    @Override
+    public String getCardIconHint(Game game) {
+        return "hexproof from multicolored";
+    }
+}