set counter 0
label .l -text even -bg 55
button .b -text click! -bg 65 -command clicked
proc clicked {} {
    global counter

    incr counter
    if {($counter % 2) == 0} {
	.l configure -text even
    } else {
	.l configure -text odd
    }
}
pack .l .b -side top -expand yes -fill both