Green is "Drill, baby, drill!", yellow is "Yeah great, I can deal", orange is "How much longer?" and red is "MOMMY!". You can't see it here really, but I'd put armpits in the red and inner biceps in yellow-orange. Two important things to mention: first, this is my own personal experience and is by no means universal - I'd really like to hear if this matches your own! And secondly, behind the knees, shins, and top of feet is based on anecdotal info - but opinion seems to be unanimous.
For the record: I think anticipated tattoo discomfort is a terrible reason to not get a tattoo. An hour or two in the studio is over before you know it and forgotten that day, but a tattoo lasts a hell of a lot longer.