A quick update on my previous post: I made up a sample of the crayon and colored pencil background on a scrap piece of cardstock so that I could see what pens would write on it. I don't have a wide variety of pens at my disposal (well, not yet, anyways), but I tried what I have and use most of the time, which are Pitt pens and Sharpies.
superfine tip -- left barely any ink, and instead scratched into the layers down to the crayon (Of course, this would be kind of cool for making interesting marks in the background, though it'd probably be better to just use a scratching tool so as not to ruin the pen.)
fine tip -- sort of scratched the surface a bit and left ink in some areas of the letters, but not enough to read the writing
medium tip -- didn't scratch the surface, but also didn't leave enough ink to make it very readable
brush tip -- didn't scratch at all and left a very nice, clear mark
ultra fine tip -- slightly scratched the surface and left just enough ink to read, but not easily
fine tip -- no scratching and left a decent mark, but it's not a smooth, clear line
As you can see, the broader the tip, the better. Only drawback is that they will smear if not left to dry for a bit. I first checked after about 10 minutes and they smeared, so I did another sample and waited about 30-40 minutes and they were fine. The difference between the Pitt brush and the fine Sharpie is that the Pitt pen leaves a nice dark mark, but the Sharpie looks a bit like the pen is starting to run out of ink. However, it is still readable, so it can depend on what look you're going for. Here's a close-up so hopefully you can see what I mean (sorry for the bit of glare):Let me know if you try this and find any other pens that work on this type of background. Have fun!