Pics are from last weekend...spotted by some Dutch guys living in A'dam !
Note that even if those pannels look nice, most others are done in a few minutes (even less sometimes) and are not really that good.
But it's worth having some spotting for sure !!
If you go to A'dam in January, take your camera with you cause if the scene is still buzy then, you'll probably see pannels running on one of the lines...
Regarding the way to spot, go to the central station and ask for a map of the underground. A'dam is not that big so it will be easy for you to find a good place to spot. Just note that you have 2 different kind of subways, the old ones (I really prefer those) and the new ones called sneltrams...
Have fun !