Some Examples To Show How It Works
Let's say the restaurant serves lunch and dinner. At 11 am, having been closed overnight, the restaurant opens for Red 3 and Yellow 2 customers only (hanging a yellow flag outside to notify the public. Strict social distancing practices are followed (it is recommended that customers make reservations). The staff dealing with the customers should all be Green 2. Staff that does not deal with the customers should either be Green 1, or should practice strict social distancing with the Green 2 segment of the staff (in no case should someone in the Red 1 or Red 2 groups be working there). At 12:30 (or as soon thereafter as the last Red 3 customer leaves), the restaurant switches to Green, changing the flag outside to plaid. It then serves lunch to Green customers under the old 'normal' conditions until 2 pm. Workers who do not have a Green designation should avoid interacting with customers (or the things they have used, e.g., unwashed dishes), and practice strict social distancing with the Green segment of the staff. The restaurant then closes and is sanitized for the dinner hour which starts at 5. Again, Red 3 and Yellow 2 customers get the first sitting and extra care, until, say, 6:30 or 7 pm (again, hang out a yellow flag, and taking reservations is strongly suggested since it prevents overcrowding or lines). After that, the restaurant switches to Green (hanging out a plaid flag, relaxing social distancing protocols) for the remainder of the night. Clean-up includes sanitation.
Having two services, one for Red 3 and Yellow 2, the other for Greens would be considerate, hanging out flags to inform people as to what type of service is being held. The service for the Red 3-Yellow 2 group should be held first (since there is no need to sanitize prior to the Green group arriving), and strict rules for social distancing should apply. Enough time should elapse between services so that the parking lot clears. The building should be sanitized prior to the next Red3-Yellow2 service.
A couple of options are available, especially for sports, like baseball, which have many games. One option is to restrict the audience to a certain type (Red3-Yellow2 or Green) for the entire game, hanging the appropriate flag outside to inform them. For a 3 game series, game one could be Red3-Yellow2, and the next two games Green. Sanitation would be required only before and during Red3-Yellow2 games. Green 2 staffing would be required for positions that interact with the public during Red3-Yellow2 games.
A second option is to designate all games as Green (hang a plaid flag outside). This allows 'business as usual'. Everyone who is not Green can follow the game on TV, the radio, or the internet. Non-green staff should avoid public areas of the stadium and practice social distancing with other staff.
A third option is to allow Red3-Yellow2 and Greens to attend the same game, but have separate entrances, hallways, and seating sections for the Red3-Yellow2 crowd (hand appropriate flags at entries to the various areas). In those areas, staff would have to be Green 2, seating would have to be safely spaced out (so reserved tickets only, with no changing of seats), access to restrooms would have to be managed, sanitation practices would have to be enhanced, and concession stands closed (all items sold by vendors in the stands). In Green sections, these practices could be relaxed so that old 'normal' protocols could be followed. This could be a good otption for stadiums that anticipate having a good number of empty seats anyway.
Since young people attend schools, and they are, generally, at low risk (Green 3), schools can operate under a plaid flag. This is the case even if teachers and staff are in the Red 3 or Yellow 2 groups, because social distancing is easily achieved between students and teachers, except in pre-school, kindergarden, and, perhaps, 1st grade (That's my guess anyway, based on a traditional model of a teacher who lectures—I haven't been in the lower grade classroom in a while, so I may be misinformed; at any rate, high schools and colleges should be able to operate in this manner even if it means doing things the 'old-fashioned' way).
Special accomodations will, of course, be necessary for Red 3 students, but it should be easier to accomodate these execeptions than to treat everyone as Red 3 (as we have been doing). Red 1 and Red 2 students, teachers, and staff should of course, follow the current stay home/distance learning model until their status changes or until someone develops a vaccine or a better plan. Students who live with Red 3 persons will, as detailed above, fall into the Red 3 category themselves, and so, even though they are not personally at risk, taking precautions with them will help keep their Red 3 housemate(s) safe.
In sizeable communities, some schools could be designated as 'Green' others as Red3-Yellow2. Students, teachers, and staff could be assigned according to the category they fall under.
If you really want to get into details about this, see this post. A first step could be to treat pedestrian traffic like vehicle traffic, and have one-way flow on each side of the street, or one-way flow on both sides of the street with the direction alternating every other block (essentially, the 'One Way' signs applying to cars would apply to pedestrian traffic as well). Add in social distancing, especially at corners, and you've made a good start.