Probably just some leftover black filament, stuck to the insides of the heat chamber, periodically coming loose and mixing in with the green plastic. It takes only a very, very small bit of black to be noticeable in just about any other color.
I suggest you try a “cold pull” to clean the nozzle. Googling will find a number of examples (and probably even videos) on how to do that.
I find the best filament for that to be Alloy910, but you can try the ngen. With Alloy910, here’s the procedure I use:
- Heat nozzle to 240, fully prime with Alloy910 by manually extruding.
- Turn off heat and allow nozzle to cool to 75c or less.
- Unlatch the extruder idler.
- Set nozzle to 160c.
- As nozzle approaches 160c (like at about 145c), begin trying to pull the filament out of the nozzle. I recommend using your other hand to hold the top of the tool head, “counterbalancing” the pull, so that you aren’t pulling against the rods/bushings. The filament should pull out a complete plug, shaped just like the inside of the nozzle.
- Repeat 1-5 until you get a clean plug.
The goal here is to pull out a solid plug of filament, shaped exactly like the interior of the nozzle, with any contaminants stuck to it. In step 5, you want to catch it just where the exterior of the filament has started to soften, but before it has really melted. Particularly with Alloy910, it will stick to any contaminants and pull them out with it.
I’ve only been successful at this using the Alloy910. I’ve seen others do it with ABS, but when I’ve tried it softens too quick and breaks rather than getting a full plug. It is pretty easy with Alloy910 and the temps mentioned above.
Here are four “plugs” I pulled out yesterday, cleaning some Red out of the nozzle. Note that each of the plugs gets gradually “cleaner”.