set counter 0
label .l -text even -bg grey55 -fg white -underline 3
button .b -text click! -bg grey65 -command clicked
proc clicked {} {
    global counter

    incr counter
    if {($counter % 2) == 0} {
	.l configure -text even -bg grey55 -fg white -underline 3
    } else {
	.l configure -text odd -bg grey85 -fg purple -underline 0
    }
}
pack .l .b -side top -expand yes -fill both