As others have said, you really can only get the answers for how to use yellow stickers from your instructor. If you look on the back of your wife’s chart, you’ll see that there are different guidelines for different women’s use of yellow stickers. Your instructor should have circled the rules that apply to your wife.
The main distinction is whether your wife is supposed to be using yellow stickers during post-peak only, or if she has graduated to pre-peak as well as post-peak yellow stickers. If she is just starting out, my guess is that she is only supposed to be using them post-peak.
My instructor told me that when deciding whether to use a yellow or green stamp post-peak, these are the guidelines:
PC or 6PC = green
6C or 8C = yellow
This is only post-peak, for me at least. Pre-peak, CM is CM, and it’s all white baby stickers. This isn’t as problematic for us since we are TTC, and we probably would have officially graduated to pre-peak yellow stickers awhile ago if we hadn’t been TTC.
As far as the essential sameness/point of change question, your instructor should be able to give you more information on this and help your wife figure out the point of change. It usually will take a few cycles. There is an easily discernible difference between PC or 6/8C and peak-type mucus, and as an aid in detecting the difference at first, your wife should compare mucus-to-mucus days and ask “Is this CM essentially the same as it was yesterday?” If yes (ie, a 6C day followed by a 6C day), it’s not a point of change. If no, (ie, a 6C day followed by an 8CL day), it can be called a point of change. I note this on my chart with a small up arrow and/or an N.
Finally, it makes a huge difference in using yellow stickers if you are avoiding pregnancy or trying to achieve pregnancy. If the difficulty in using yellow stickers means long periods of abstaining, it might be a reason for you and your wife to return to prayer to discern how serious your reasons for avoiding pregnancy at this time really are. That’s what happened for us, anyway.
Oh, one more thing. Long mucus cycles are something you should discuss with a physician, preferably one certified in Naprotechnology. There are ways of shortening the mucus cycle, and extended CM can point to different fertility/women’s health issues.