Some notes about the system:
1) It uses the parameters from the application the station designated as their post-transition facility on their form 387. In other words, we rely on the stations to have filled out a form correctly. There are some cases where the form is incorrect. (thanks itsthemultipath!) We're looking into fixing the system on a case-by-case basis when from 387 doesn't agree with reality, but it's not possible to manually verify the parameters for 1800+ stations, so if you notice something, let me know (PM me). This system does not take the last-in-time application if there is a Form 387 on file (which is what TVFool appears to do) because this is often NOT the appropriate post-transition facility.
2) It uses 1km terrain sampling, whereas TVFool uses 100m terrain sampling. This was done for speed of the system (which as you can see is already slow enough). This doesn't make a difference for most people, but if you're in the type of terrain where a hill can be completely erased by 1km terrain sampling, then it's going to be overly optimistic.
3) It uses the exact same algorithm as TVFool, the "ILLR" algorithm. The colors are slightly different. I'm not sure what their thresholds are, but on the FCC one, red is below the band-dependent reception threshold listed in the rules and DTV planning factors. Orange is 0-15dB above that. Yellow is 15-35 db above the threshold, and green is >35dB above the threshold.
4) As a general rule of thumb, indoor antennas should be able to receive green stations, attic antennas should be able to receive yellow stations, rooftop antennas are necessary for orange stations, and red stations will require a very large highly directional rooftop antenna. Of course, your mileage may vary, especially on indoor antennas.