• Would it even give a consistent or desired color in the finished print, or not fade/wash away?

• Would the ink damage the hotend or cause problems with the extrusion? 

• Would it leave behind too much residue that would continue to add color for too long?

Has anyone tried something like this?

I have messed around with this a few times and had some interesting results.
Now that I have a bit more time, I can answer a couple questions:
A1: It would be fairly consistent, but it would depend on what type/color filament vs what color marker you use for this. Just remember that once the combination of filament+colorant enter the hotend, you have lost all control of the mixture, so it can be tricky to get a more controlled effect.
A2: It is possible to get a partial clog. While swapping nozzles shortly after my experiments, I found some residue that resembled a powered soot. This was some ink left over, and I did have a slight clog. With an all metal hotend like an E3D, it was very simple to clear it out and I was back up and printing in minutes.
A3: When using a black marker on red and orange filament, I noticed black or at least a darker hue of the filament's color for about 150mm after the last section of colored filament went in. But this can give a pretty cool result if you are patient and don't over color.

I would not recommend using this technique for an entire roll of filament, but if you just want to add a few color features that only a dual extruder can create, then this can be a fun experiment. If I can ever get my imgur app to work, I will share a print I did with this approach.

http://www.thingiverse.com/thing:42056 for dual markers

http://www.thingiverse.com/thing:5570 for single marker

Thank you!

http://www.thingiverse.com/thing:5570 for single marker

• Would it even give a consistent or desired color in the finished print, or not fade/wash away?

• Would the ink damage the hotend or cause problems with the extrusion? 

• Would it leave behind too much residue that would continue to add color for too long?

Has anyone tried something like this?