I'm not speaking for the creators of the site... but I think their intention was to simplify the classifications. It would serve no purpose to have 20 different color codes when there are only 50 states. The four (five?) general categories give a decent enough idea of how states' laws work, and people can and should look at the individual state pages to see details.
ETA: As for may issue v. shall issue, another idea that might be lost on newer members (I mean no offense) is that all permits are a violation of rights, no matter how freely they are given out. One of the main purposes of open carry is that in many states, a permit is not required. The way I look at it, the states that allow open carry without a license are all "good", so to speak, and the states that require a license are equally not so good from a Constitutional perspective, regardless of if that states approves 99% or applications for permits of .09% of applications.
Also, welcome to OCDO! Happy open carrying!