Mana updates

+ calling subtraction will now throw an exception if you try and use
more mana than is available. This is better than setting it to 0.
Setting to 0 impose that you should still be allowed to perform the
action.
+ updated subtraction test to check for exception
+ subtractionCost() will not allow using mana that is not available,
same as subtract()

Mage.Tests/src/test/java/org/mage/test/mana/ManaTest.java

@@ -1,5 +1,6 @@
 package org.mage.test.mana;
 
+import junit.framework.Assert;
 import mage.Mana;
 import mage.constants.ColoredManaSymbol;
 import mage.constants.ManaType;
@@ -434,6 +435,8 @@ public class ManaTest {
     @Test
     public void shouldNotSubtractLessThan0() {
         // given
+        expectedException.expect(ArithmeticException.class);
+        expectedException.expectMessage("You can not subtract below 0");
         Mana thisMana = new Mana(2, 2, 2, 2, 2, 2, 2);
         Mana thatMana = new Mana(10, 1, 1, 1, 10, 1, 1);
 
@@ -441,13 +444,57 @@ public class ManaTest {
         thisMana.subtract(thatMana);
 
         // then
-        assertEquals(-8, thisMana.getRed());
-        assertEquals(1, thisMana.getGreen());
-        assertEquals(1, thisMana.getBlue());
-        assertEquals(1, thisMana.getWhite());
-        assertEquals(-8, thisMana.getBlack());
-        assertEquals(1, thisMana.getColorless());
-        assertEquals(1, thisMana.getAny());
+    }
+
+
+    @Test
+    public void shouldNotAllowMinusSubtractionCost() {
+        // given
+        expectedException.expect(ArithmeticException.class);
+        expectedException.expectMessage("You can not subtract below 0");
+        Mana thisMana = new Mana(2, 2, 2, 2, 2, 2, 2);
+        Mana thatMana = new Mana(10, 1, 1, 1, 10, 1, 1);
+
+        // when
+        thisMana.subtractCost(thatMana);
+
+        // then
+
+    }
+
+
+    @Test
+    public void shouldUseExistingManaToPayColorless() {
+        // given
+        Mana available = new Mana();
+        available.setRed(7);
+
+        Mana cost = new Mana();
+        cost.setRed(4);
+        cost.setColorless(2);
+
+        // when
+        available.subtractCost(cost);
+
+        // then
+        assertEquals(1, available.getRed());
+    }
+
+
+    @Test
+    public void shouldThrowExceptionOnUnavailableColorless() {
+        // given
+        expectedException.expect(ArithmeticException.class);
+        expectedException.expectMessage("Not enough mana to pay colorless");
+        Mana available = new Mana();
+        available.setRed(4);
+
+        Mana cost = new Mana();
+        cost.setRed(4);
+        cost.setColorless(2);
+
+        // when
+        available.subtractCost(cost);
     }