Personally I absolutely hate the taste of fondant sheets & so does everyone else I know.
Personal tastes aside I do use fondant sheets on request, I would only recommend them if you are already doing a fondant cake & only for big images. But I would actually use Wafer Card for the large images (or wafer paper works well too)
For me it is wafer paper all the way & Vanilla over unflavoured all the time, I even use vanilla on savoury items (but then I am a little obsessed with vanilla )
We are able to get images nice and clear on wafer paper. For larger wafer designs when I want them to stand up I will use wafer card. These work best on the side of large cakes when you want the image to stick up over the top of the cake slightly.
Okay, but you may be a little biased, I think. On a more serious note, a question: what is a "fondant sheet"? We use the terms "frosting sheet" or "icing sheet" here in the US, and both are typically made with tapioca starch and sugar; they are not thin layers of fondant. Also, what are "wafer cards"? We typically only see wafer paper here, which is pretty sheer and brittle and allows any color behind it to show through.
Fondant sheets are the same as frosting sheets. Just what we call them here in the UK.
Wafer Card is thicker smooth on both sides and is ideal for a large images that you would like to stand up slightly. (comes in Vanilla or Unflavoured too) the colour has not come through on everything I have used ours on so far, but I haven't tried them on cookies with chocolate icing yet (cupcakes & cakes were fine)
Depending on the Wafer Paper some can be very sheer & very thin about the thickness of plain paper or even thinner sometime (usually I found them to be a bit blotchy too) those will show through pretty much any colour, usually these are smaller than A4 as well.
Then there are other papers which are a bit thicker (which I go for) usually these are not blotchy sometimes you might find an odd sheet that is though, these are usually A4 size.
For Wafer paper a lighter colour icing for cookies/biscuits is always preferred because then it will not interfere with the image on top, but this does depend on the image (if it is a mainly dark image you can go for stronger colours for the icing). If you have a chocolate icing (and an image that is not dark colours) it is best to do a white icing (or light colour) just under the image, then you could give it a border in chocolate icing.
When storing wafer paper you need to store them in an airtight container (we vacuum pack everything) then place that container in a cool (room-temperature) dark space, do not put in the fridge or freeze, although you can put your wafer paper once on cookies (or any other edible creation) in the refrigerator but you will not be able to move the wafer paper after this point as it will be brittle so can break.
If you find your wafer paper has become brittle before you go to use it (this shouldn't happen if it is kept how I mentioned above) simply place your wafer paper in a container (doesn't have to be airtight for this bit) then place that container with your wafer paper into your airing cupboard. Leave them there for about 30 minutes to 1hr (or longer if you decide to) you will then find the paper is more flexible & easier to apply.
Once you take your wafer paper out of the airing cupboard place straight into an airtight container, if you leave your wafer paper lying out in the air it will become brittle again after an hour or so, it is best to keep your wafer paper in the air tight container until the moment you want to apply them to the cookie (or other creation)
If you find your wafer paper (or card if it has been left out) start to curl get some heavy hard backed books then either place the wafer paper inside the book right in the middle of the book (making sure that it is not poking out of the book) or you can place them between 2 hard backed books (heavier the better) then you can either place them in the airing cupboard for 30mins - 1hr (or more) or leave them out on your side or table as the wafer paper is covered by the books so there is not much air to get to them (if its winter & your house is cold place them in the airing cupboard)
Fondant sheets can work well for large designs & some medium sizes in simple shapes (rectangles squares, circles)
I think I have just had far to many problems with Fondant sheets for me ever to like them.
I found the ink takes forever to dry (almost 24hrs once), it breaks easily so can only be used for simple shapes. The taste as well is horrid (but that's my personal taste & families).
I am not really a fan of the taste of unflavoured wafer paper/card either but that may be because I am too used to vanilla now hehe