rainbow.sh

PERIOD=300
AMPLITUDE=6000
O1=$((PERIOD/3))
O2=$((O1*2))

UUID=$(osascript -e "tell application \"iTerm2\"
  get current session of current window
end tell" | cut -d' ' -f3 -f6 -f10)

set-background() {
  osascript -e "tell application \"iTerm2\"
    tell window id $3
      tell tab $2
        tell session id \"$1\"
          set background color to $4
        end tell
      end tell
    end tell
  end tell" 2>/dev/null || exit
}

if [ -z ${1+x} ]; then
  LOOP=false
else
  LOOP=true
fi

c() {
  bc -l <<< "scale=0;m=$1%$PERIOD;scale=20;if(m>$O2){0}else{n=(c(12*a(1)*m/$PERIOD+4*a(1))+1)*$AMPLITUDE;scale=0;n/1}"
}
while true; do
  for ((i=1;i<=PERIOD;i++)); do
    set-background $UUID "{$(c $i), $(c $((i+O1))), $(c $((i+O2))), 0}"
  done
  $LOOP || break
done

This uses an unfortunately large amount of CPU right now...