set preferred size of colour panel

assignments/a2/src/main/java/Sidepane.java

@@ -46,6 +46,7 @@ class Colours extends JPanel
   public Colours()
   {
     this.setLayout(new GridLayout(0, 2));
+    this.setPreferredSize(new Dimension(100, 225));
     red = new JButton("Red");
     green = new JButton("Green");
     blue = new JButton("Blue");