Fix: Only calculate ColorsUsed once

* This commit provides the proper fix mentioned in
  cd423d9d
* We still keep the caching implemented in
  cd423d9d

src/SparseTransform/DataStructures/AdjacencyGraph.cs

@@ -45,6 +45,7 @@ namespace SparseTransform.DataStructures
             }
         }
 
+        private int _colorsUsed = 0;
         /// <summary>
         /// Number of colors used for coloring.
         /// </summary>
@@ -52,20 +53,24 @@ namespace SparseTransform.DataStructures
         {
             get
             {
-                List<int> colors = new List<int>();
-                if (Colored)
+                if (this._colorsUsed == 0)
                 {
-                    foreach (GraphNode node in _nodes)
+                    List<int> colors = new List<int>();
+                    if (Colored)
                     {
-                        int color = node.Color;
-                        // Do not check for != 0 since we already know everything here is colored
-                        if (!colors.Contains(color))
+                        foreach (GraphNode node in _nodes)
                         {
-                            colors.Add(color);
+                            int color = node.Color;
+                            // Do not check for != 0 since we already know everything here is colored
+                            if (!colors.Contains(color))
+                            {
+                                colors.Add(color);
+                            }
                         }
                     }
+                    this._colorsUsed = colors.Count;
                 }
-                return colors.Count;
+                return this._colorsUsed;
             }
         }
 
@@ -79,7 +84,7 @@ namespace SparseTransform.DataStructures
 
         public void AddEdge(int i, int j)
         {
-            if(i != j)
+            if (i != j)
             {
                 GraphNode iNode = AddNode(i);
                 GraphNode jNode = AddNode(j);

src/SparseTransform/DataStructures/BipartiteGraph.cs

@@ -88,6 +88,7 @@ namespace SparseTransform.DataStructures
             }
         }
 
+        private int _colorsUsed = 0;
         /// <summary>
         /// Number of colors used for coloring. Works if either or both sides are colored
         /// </summary>
@@ -95,32 +96,38 @@ namespace SparseTransform.DataStructures
         {
             get
             {
-                List<int> colors = new List<int>();
-                if (LeftColored)
+                if (_colorsUsed == 0)
                 {
-                    foreach (GraphNode node in _leftNodes)
+                    List<int> colors = new List<int>();
+                    if (LeftColored)
                     {
-                        int color = node.Color;
-                        // Do not check for != 0 since we already know everything here is colored
-                        if (!colors.Contains(color))
+                        foreach (GraphNode node in _leftNodes)
                         {
-                            colors.Add(color);
+                            int color = node.Color;
+                            // Do not check for != 0 since we already know everything here is colored
+                            if (!colors.Contains(color))
+                            {
+                                colors.Add(color);
+                            }
                         }
                     }
-                }
-                if (RightColored)
-                {
-                    foreach (GraphNode node in _rightNodes)
+                    if (RightColored)
                     {
-                        int color = node.Color;
-                        // Do not check for != 0 since we already know everything here is colored
-                        if (!colors.Contains(color))
+                        foreach (GraphNode node in _rightNodes)
                         {
-                            colors.Add(color);
+                            int color = node.Color;
+                            // Do not check for != 0 since we already know everything here is colored
+                            if (!colors.Contains(color))
+                            {
+                                colors.Add(color);
+                            }
                         }
                     }
+                    this._colorsUsed = colors.Count;
                 }
-                return colors.Count;
+
+                return _colorsUsed;
+
             }
         }