This design was inspired by theorbtwo's marker holder, except I wanted something open that didn't need to be fastened shut. This part should help rapidly color a length of filament feed stock with a few swipes.
This is designed to take a fine point sharpie marker and paint a 3mm filament.
Print. The round hole prints horizontally and I was too lazy to put in the teardrop top, so it sags a bit. It might need some cleaning out, but the three I've printed seemed OK after minimal cleaning.
The plastic tip of the marker should fit snug in the small hole, with about 1mm sticking through. This should leave a gap on either side of the marking tip, placing filament near the center of the side of the tip. For full saturation color, you may need to pass over the filament several times, back and front. (Shifting the filament to the opposite gap makes this easy.)
If your marker or your filament is a different size, you may need to adjust the parameters of the model. Parameters penbase and pentol control the diameter of the tapered hole for the marker handle, pentip the small hole. If you use a thinner filament, you may want to increase pentiplen (by 1mm?) so that the entire plastic portion of the tip is inside the block.