If you have to use a dyed towel, don't despair. Any color that bleeds onto the concrete is fairly easily removed using bleach and a stiff plastic brush.
If you're making a larger piece, you might consider adding some kind of reinforcement to the concrete mixture. This could be a wire mesh pre-form, or stiffening elements that are inserted after the bottom layer is tamped, but before the walls are packed down. It might also be possible to add fibrous material to the bulk concrete mix to improve its tensile strength, but I haven't actually done any experiments to that end as of yet.
There are lots of different concrete mixtures out there, and you should feel free to experiment with other aggregates, even unusual or novel ones. I've made a couple of pots using vermiculite in place of sand in this recipe, which results in a finer, smoother surface texture.
Be careful with this process--once you start seeing discarded styrofoam as a source of inspiration, you may find it piling up faster than you can churn out the pots!