Here is the TmuxNotifier and the .rspec changes needed to use it.

.rspec

--color -f TmuxNotifier

tmux_notifier.rb

require 'rspec/core/formatters/progress_formatter'

class TmuxNotifier < RSpec::Core::Formatters::ProgressFormatter
  def initialize(output)
    @tmux_message = ""
    @write_to_tmux = true
    super
  end

  def dump_summary(duration, example_count, failure_count, pending_count)
    summary = pluralize(example_count, "example")
    summary << ", " << pluralize(failure_count, "failure")
    summary << ", #{pending_count} pending" if pending_count > 0

    system "tmux set quiet on"

    if pending_count > 0
      system "tmux set status-right-bg yellow"
    elsif failure_count > 0
      system "tmux set status-right-bg red"
    else
      system "tmux set status-right-bg green"
    end

    @tmux_message = summary + @tmux_message

    super
  end

  def dump_failure_info(example)
    if @write_to_tmux
      exception = example.execution_result[:exception]

      @tmux_message << "  Failure/Error: #{read_failed_line(exception, example).strip} "

      exception.message.each_line do |line|
        @tmux_message << "#{line.gsub("\n","").strip} "
      end

      @write_to_tmux = false
    end

    super
  end

  def close
    system "tmux set status-right-length #{@tmux_message.length + 5}"

    system "tmux set status-right '#{@tmux_message.gsub('`','').gsub("'",'')}'"
    super
  end
end

Not sure if the logic here is what you intended, but when you have failing and pending specs you'll get a yellow background.

In rgrmux I've changed this so that the background is red if there are any failing specs.