Well, the printer is fully addressable in X/Y coordinates with a 0.2mm accuracy, so it's just a matter of drawing horizontal lines in the right color, moving back and down a little bit, then draw another horizontal line.
The problem is mostly that with all the back and forth, you end up having some inaccuracies and things don't always align properly... and it eats up a crapton of ink!
Nope, can't do yellow, or brown, or anything really, these colors were chosen to match the type of ball point pen you would normally find, it was not really designed to make high color pictures.Notice: it is bad they choose R/G/B pens because you can't do subtractive color addition with them. Did you succeed in producing correct yellow color with it? ( by introducing a little offset between the R and G pass so they each print over white paper for instance).
But I guess what you did with your picture conversion program could indeed probably be done to convert pictures in some "pointiliste" style, would be interesting to try