As others have said, it is state specific and probably largely not followed even in the states that have it on the books since its probably not well known.
If anyone knows of a complete list of states that have it on the books, that would be a great reference.
But I do remember seeing other people mention getting ticketed for it, and I think it was California specifically. I think it might have been a weigh station or other DOT kind of officer that was probably more familiar with the commercial side of the house that wrote that ticket.
I don't think my home state of Colorado cares, but I am traveling to SD soon, and I found a reference that they care, so I am trying to paint my bus quickly in the next couple of weeks.
You would think that if my state doesn't care then they should respect that through reciprocity, but I do remember in my younger days while in the military in TX, that the TX cops loved writing tickets for some specific TX laws, didn't matter what state your plates were from. So maybe legally they should recognize it, but are you going to travel back to a state you are visiting and hire a lawyer for a (hopefully) less than $100 ticket? In the case of SD, they bring in volunteer officers from all over for Sturgis so who knows which officers are familiar with which laws, so I assume its better to just try to get some color on it other than yellow.
As far as legal remedies goes, National School Bus Yellow is a very specific color, which I think if you have a law it has to be specific. My bus currently has multiple shades of yellow from Fading, and each bus company has a slightly different shade of School Bus Yellow, so if you had an expensive lawyer, he might be able to argue that your bus was yellow, but they had no way to prove it was specifically NSBY. See above, about it not being worth it to fight a nonmoving violation that has a low fine.