diff --git a/rbt.py b/rbt.py
index 9f9203a..3adbf0d 100644
--- a/rbt.py
+++ b/rbt.py
@@ -61,7 +61,7 @@ class RBTree(RankedTree[T]):
             # Case 1
             # ======
             # z’s uncle y is red
-            pp.rank += 1
+            pp.rank += Colour.Black
             z = pp
         elif z == right_child:
             # Case 2
@@ -105,33 +105,33 @@ class RBTree(RankedTree[T]):
         rotate_left: RotateFunction,
         rotate_right: RotateFunction,
     ) -> Tuple[Optional[Node[T]], Optional[Node[T]]]:
-        if Node.difference(w) == 0:
+        if Node.difference(w) == Colour.Red:
             # Case 1
             # ======
             # x’s sibling w is red
             rotate_left(tree=self, x=parent)
             w = right(parent)
 
-        if Node.differences(w) == (1, 1):
+        if Node.differences(w) == (Colour.Black, Colour.Black):
             # Case 2
             # ======
             # x’s sibling w is black, and both of w’s children are black
-            parent.rank -= 1
+            parent.rank -= Colour.Black
             x = parent
         else:
             # Case 3
             # ======
             # x’s sibling w is black,
             # w’s left child is red, and w’s right child is black
-            if Node.difference(right(w), w) == 1:
+            if Node.difference(right(w), w) == Colour.Black:
                 rotate_right(tree=self, x=w)
                 w = right(parent)
 
             # Case 4
             # ======
             # x’s sibling w is black, and w’s right child is red
-            parent.rank -= 1
-            w.rank += 1
+            parent.rank -= Colour.Black
+            w.rank += Colour.Black
             rotate_left(tree=self, x=parent)
             x = self.root