set choice red
set index 0
set select on
set list {red green blue white brown grey yellow}
set len [llength $list]
foreach i $list {
    radiobutton .$i -text $i -variable choice -value $i -bg grey77
    pack .$i -side left -fill y
}
label .l -textvariable choice
pack .l -side left -expand y -fill both
proc tick {} {
    global choice index len list select

    if {"$select" == "on"} {
	set choice none
	set select off
    } else {
	set select on
	incr index
	if {$index == $len} {
	    set index 0
	}
	set choice [lindex $list $index]
    }
    after 1000 tick
}
tick