If you use the 2 bucket method, the wash solution should and will stay clean enough to be reusable. The water softeners in it don't let dirt particles to float in the mix; they drop to the bottom of the bucket.
If you clean a not too dirty vehicle and end up with a relatively clean wash solution, it is not worse than it 'd be using it on a dirtier car - from the halfway point during the wash. The latter solution will be definitely dirtier at the end.
Besides, you can filter it through a coffee filter, no problem. Filter in a funnel and go.
If you encounter heavily soiled parts (underside of rear bumpers, rocker panels, etc.) use a coin-op to rinse the rough grit down. You can use it on surprisingly dirty car as well, but use LOTS of wash solution. Lift plenty of water to the area, with your mitt barely touching the surface. Rinse the mitt. ONR again. Rinse and repeat as necessary. Work on small areas and don't bother with drying until the area is clean. If it is still necessary (wind dried the surface, or whatever), go over the cleaned area again then dry. When I use ONR and the weather is not windy nor too hot, I dry only after doing 3-4 panels, i.e. a complete side.