I am in the middle of a lengthy project that involved numerous skills, one of which is painting panels of wood. I've got a roller and tray, but this is the first time I have had my own painting equipment to look after. How do experienced painters tell when the roller is better thrown out than washed? I can still get a nap on the roller, but it's difficult to tell what colour it was as I just can't wash all the paint out.
If you can't get all the paint out then that's a good time to think about replacing it.
The problems you might get are some paint flecks from the old paint coming off onto the newly painted surface - which will ruin the finish, or some of the old colour leaking into the new.
I wouldn't worry about the colour of the roller material itself and it is a good sign that you can still get a nap on the roller.
It may be considered wasteful, but I only use the roller for repeat coats of the same color. If I am applying 2 or 3 coats, I will reuse the same roller. If I am painting a different color, I always use a new roller, so I make sure the color is true. I can never seem to get all the paint out of the roller. I would rather spend the money on a new roller than have the paint messed up.
When I reuse the roller, I wrap it in plastic wrap. I have also placed rollers in the freezer after wrapping them, so the paint cannot dry out.
The reason for using a new generic roller is if you don't clean it perfectly you'll have remnants that cause patterns to appear in the way it applies paint to the surface. After using a roller enough times it will start to leave more of a textured look than a smooth clean coating. Ideally you would wrap it in some sort of plastic wrap or air tight container to prevent the paint from drying out on it giving you the opportunity to use the same color again the next day for a 2nd coating.