You use System Reflection to dynamically load a control. If the DLL is named "SpecControls.DLL" and the class you want is "SpecControls.ColorControl", then use this code.
// load the assembly
System.Reflection.Assembly assembly = Assembly.LoadFrom("SpecControls.DLL");
// get the type
Type t = assembly.GetType("SpecControls.ColorControl");
Control c = (Control)Activator.CreateInstance(t);
parent.Controls.Add(c);