Groovy SwingBuilder : button to change the color of a panel

Why does this code fail ? I want to change the color of one panel in a series of several panels, dynamically constructed (total number of panels not known beforehand).

For some reason, this code works when referencing the name of a particular panel (for example 'panel2'), but not when I refer to it dynamically ('panelID').

import groovy.swing.SwingBuilder
import javax.swing.WindowConstants as WC
import javax.swing.JOptionPane
import javax.swing.BoxLayout as BXL

swing = new SwingBuilder()
frame = swing.frame开发者_高级运维(title:'test',
    defaultCloseOperation:WC.DISPOSE_ON_CLOSE) {

    panel(id:'mainPanel'){
        def panelID 

        (1..6).each {
            panelID = 'panel' + it

            panel(alignmentX: 0f, id: panelID , opaque:true ,background : java.awt.Color.GREEN){
                label('description') 
                textField(id: "description$it", text: panelID, columns: 70 )
                button(id: "button$panelID", text: panelID, actionPerformed : {
                    panelID.background = java.awt.Color.RED
                    panelID.repaint()                       
                })
            }
        }
        boxLayout(axis: BXL.Y_AXIS)

        panel(id:'secondPanel' , alignmentX: 0f){                       
            button('Quit', actionPerformed:{
                dispose()
            })
        }
    }       
}
frame.pack()
frame.show()

To get the element based on it's ID, you need to access the ID as a parameter of the SwingBuilder, like so:

import groovy.swing.SwingBuilder
import javax.swing.WindowConstants as WC
import javax.swing.JOptionPane
import javax.swing.BoxLayout as BXL

swing = new SwingBuilder()
frame = swing.frame(title:'test', pack:true, visible:true, defaultCloseOperation:WC.DISPOSE_ON_CLOSE) {
    panel(id:'mainPanel'){
        (1..6).each { num ->
            def panelID = "panel$num"
            def pane = panel( alignmentX:0f, id:panelID, background:java.awt.Color.GREEN ) {
                label('description') 
                textField(id: "description$num", text:panelID, columns: 70 )
                button(id: "buttonpanel$num", text:panelID, actionPerformed : {
                    swing."$panelID".background = java.awt.Color.RED
                })
            }
        }
        boxLayout(axis: BXL.Y_AXIS)

        panel(id:'secondPanel' , alignmentX: 0f){                       
            button('Quit', actionPerformed:{
                frame.visible = false
            })
        }
    }       
}