Do certain markers work better on felt?
I tried an experiment on a square of random tan felt I had sitting around. I chose the letter B because it has a good mix of straight and curved lines, so I could also get an idea of how easy a particular marker was to use.

Clockwise, from top left:


Permament Marker (Sharpie)
Washable Marker (Faber Castell)
Fabric Marker (Marvy)
Fabric Paint (Tulip)






Permanence: The washable marker easily transferred color from fabric to finger. To my surprise, even the permanent marker could be smeared if rubbed with a thumb. The fabric paint and fabric marker didn't transfer color to my fingers, though.

One additional note: Because of the nature of felt, the surface threads can be rubbed around in such a way that the letter looks smeared -- even the fabric marker version. The fabric paint, however, also "glued" the threads in place and so the letter couldn't be distorted.

Ease of Use: This was sort of a tie. The fabric paint was the easiest, because I only needed one pass for each line I made. However, the felt really wicks up paint, and so it took a couple attempts to figure out how much to apply (a thin paint line expands and becomes thicker as it dries). See the purple blob to the left of the letter? That was attempt #1 :)

Bleedthrough: Fabric paint and permanent marker showed through the most; all could be seen to some extent, though.