You are trying to change the colour of an int I not the listbox item.
Use owner-draw mode of the ListBox. Select your ListBox in design-mode and change DrawMode property to OwnerDrawFixed. Now attach a handler to DrawItem event and then use Graphics class's methods to draw your string in any color or font you like. An example of what you need to do in DrawItem would be:
private void listBox1_DrawItem(object sender, DrawItemEventArgs e)
MyListBoxItem item = listBox1.Items[e.Index] as MyListBoxItem;
if (item != null)
{
e.Graphics.DrawString(
item.Message,
listBox1.Font,
new SolidBrush(item.ItemColor),
0,
e.Index * listBox1.ItemHeight
);
}