DefaultTableCellRenderer not working

I am trying to change the color of the cells of the third row of my JTable if they have a value. I read that a good way to do this is using a table cell renderer. However, it just seems not to be doing anything! Here there is my RENDERER code:

public class RenderTablaPrestamos extends DefaultTableCellRenderer{

   @Override
   public Component getTableCellRendererComponent (JTable tabla, Object valor,
                                                boolean isSelected, boolean hasFocus,
                                                int row, int col){
    JLabel celda = (JLabel) super.getTableCellRendererComponent(tabla, valor, isSelected, hasFocus, row, col);

        if(valor instanceof Integer){
            Integer v=(Integer)valor;
            if(col==3){
                if(valor!=null){
                     celda.setBackground(Color.red);
                }
                else{
                    celda.setBackground(Color.WHITE);
                }
            }
            else{
                celda.setBackground(Color.WHITE);
            }
        }
    return celda;
    }
}

Here there is how I use my renderer:

tablaUsuariosPrestamos.setDefaultRenderer(Object.class,new RenderTablaPrestamos());

Here there is a picture of my JTable (I do not think the model code would be any useful as it is kinda long):

I do not think it does anything to do with the if clausules, as I commented them and it did not work either.

Where am I going wrong?

Use

for (int i = 0; i < tabla.getColumnCount(); i++) {
    tabla.getColumnModel().getColumn(i).setCellRenderer(new RenderTablaPrestamos());
}

instead of

tablaUsuariosPrestamos.setDefaultRenderer(Object.class,new RenderTablaPrestamos());