How to update multibinding manually
For a multibinding, the binding expression is a MultiBindingExpression, which inherits from BindingExpressionBase, but not from BindingExpression. So GetBindingExpression returns null for a multibinding. Instead you can use BindingOperations.GetMultiBindingExpression:
MultiBindingExpression b = BindingOperations.GetMultiBindingExpression(colorRectangle, Rectangle.FillProperty);