Reflection actually demanded ImmutableList so revert to that

src/com/massivecraft/massivecore/item/ConverterListImmutable.java

@@ -0,0 +1,58 @@
+package com.massivecraft.massivecore.item;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableList.Builder;
+
+public class ConverterListImmutable<EX, EY> extends Converter<Iterable<EX>, ImmutableList<EY>>
+{
+	// -------------------------------------------- //
+	// FIELD
+	// -------------------------------------------- //
+	
+	private final Converter<EX, EY> converterElement;
+	public Converter<EX, EY> getConverterElement() { return this.converterElement; }
+	
+	// -------------------------------------------- //
+	// CREATE
+	// -------------------------------------------- //
+	
+	public ConverterListImmutable(Converter<EX, EY> converterElement)
+	{
+		this.converterElement = converterElement;
+	}
+	
+	// -------------------------------------------- //
+	// OVERRIDE
+	// -------------------------------------------- //
+	
+	@Override
+	public ImmutableList<EY> convert(Iterable<EX> exs)
+	{
+		// Null
+		if (exs == null) return null;
+		
+		// Create
+		Builder<EY> eys = ImmutableList.builder();
+		
+		// Fill
+		for (EX ex : exs)
+		{
+			EY ey = null;
+			try
+			{
+				ey = this.getConverterElement().convert(ex);
+			}
+			catch (Throwable t)
+			{
+				t.printStackTrace();
+				continue;
+			}
+			
+			eys.add(ey);
+		}
+		
+		// Return
+		return eys.build();
+	}
+	
+}

src/com/massivecraft/massivecore/item/ConverterToColors.java

@@ -2,7 +2,7 @@ package com.massivecraft.massivecore.item;
 
 import org.bukkit.Color;
 
-public class ConverterToColors extends ConverterList<Integer, Color>
+public class ConverterToColors extends ConverterListImmutable<Integer, Color>
 {
 	// -------------------------------------------- //
 	// INSTANCE & CONSTRUCT

src/com/massivecraft/massivecore/item/WriterFireworkEffectColors.java

@@ -1,11 +1,12 @@
 package com.massivecraft.massivecore.item;
 
+import com.google.common.collect.ImmutableList;
 import org.bukkit.Color;
 import org.bukkit.FireworkEffect;
 
 import java.util.List;
 
-public class WriterFireworkEffectColors extends WriterAbstractFireworkEffect<List<Integer>, List<Color>>
+public class WriterFireworkEffectColors extends WriterAbstractFireworkEffect<List<Integer>, ImmutableList<Color>>
 {
 	// -------------------------------------------- //
 	// INSTANCE & CONSTRUCT
@@ -37,9 +38,9 @@ public class WriterFireworkEffectColors extends WriterAbstractFireworkEffect<Lis
 	}
 	
 	@Override
-	public List<Color> getB(FireworkEffect cb, Object d)
+	public ImmutableList<Color> getB(FireworkEffect cb, Object d)
 	{
-		return cb.getColors();
+		return (ImmutableList<Color>) cb.getColors();
 	}
 	
 }

src/com/massivecraft/massivecore/item/WriterFireworkEffectFadeColors.java

@@ -1,11 +1,12 @@
 package com.massivecraft.massivecore.item;
 
+import com.google.common.collect.ImmutableList;
 import org.bukkit.Color;
 import org.bukkit.FireworkEffect;
 
 import java.util.List;
 
-public class WriterFireworkEffectFadeColors extends WriterAbstractFireworkEffect<List<Integer>, List<Color>>
+public class WriterFireworkEffectFadeColors extends WriterAbstractFireworkEffect<List<Integer>, ImmutableList<Color>>
 {
 	// -------------------------------------------- //
 	// INSTANCE & CONSTRUCT
@@ -37,9 +38,9 @@ public class WriterFireworkEffectFadeColors extends WriterAbstractFireworkEffect
 	}
 	
 	@Override
-	public List<Color> getB(FireworkEffect cb, Object d)
+	public ImmutableList<Color> getB(FireworkEffect cb, Object d)
 	{
-		return cb.getFadeColors();
+		return (ImmutableList<Color>) cb.getFadeColors();
 	}
 	
 }