set choice 0
set c 0
foreach i {red green blue white brown yellow} {
    radiobutton .$i -text $i -variable choice \
	    -value [expr $c % 2] -bg grey77
    pack .$i -side left -fill y
    incr c
}
label .l -textvariable choice
pack .l -side left -expand y -fill both
proc tick {} {
    global choice

    if {$choice} {
	set choice 0
    } else {
	set choice 1
    }
    after 500 tick
}
tick