Painted floors and "Rustic" don't go together (IMHO). I agree with Steve, painting them, you may regret later as it is almost impossible to completly remove the paint and apply a varnish at a later date.
However, my brother did a floor in his house using a gray floor paint and then mixed a glazing liquid with a floor varnish, which he then brushed over the floor to give it a "dirty" look. Once the glazing was dry, he varnished over it with a semi-gloss floor varnished. The end result was quite impressive. If you wish to try this, I'd recomend that you find some wood that is close in texture to your floor and try it on the board first. Once you start on the floor, your kind of commited to it.