OK, you've asked for an honest opinion and I've given it some thought.
Firstly, I think the idea has potential and that there would be a market out there for sure. Most ttc'ers will buy everything at some stage or another, but then narrow their choice of product down to a selection that suits their individual needs.
I think a dark coloured pad is a great idea .. but definitely not a shade of red as this could obscure any spotting during the cycle. My favoured choice would be black as I find black undies the best for 'tell tale signs'. (However see
note* below).
Not sure how the depth markers would work because the amount of CM is quite subjective from person to person, so say my amount of EWCM could be miniscule in comparison to others, but it is still fertile fluid no matter how small an amount.
You also need to take into consideration that CM does not always make it to the vaginal opening, so could get missed without internal examination (sorry about this party pooper, but it does need to be considered).
I read somewhere that the pattern your CM forms on your underwear is also an indicator of what type of fluid your are producing. For example, if it is circular in shape it is considered fertile because it is thicker and tends to blob and if it is longer and more rectangular on your underwear, then it is considered more watery and less fertile. This pattern could be part of the 'read your liner' sales pitch.
Now, my vote is a yes for the product but I know I would not purchase it (excuse my honesty).
*Note: I think black is a good colour because it is the best contrast to white/cream, but pink or red spotting tends not to show up very well on it. So perhaps a darkish shade of green or blue would be preferable.
As for the name of the product, I vote against the word 'PAD' because I just hate the word in this context and think an acronym would be great .. you know like the OPK's and HPT's etc. Say something like CFR (cervical fluid reader) or CPL (cervical panty liner) or RTS (read the signs) etc.
I think CUM (check underwear mucous) might be a non starter

don't you?