The hue 5B in the Munsell system is not supposed to be some idealized “blue”. The “B” is just a mnemonic device: breaking the 100 hues which make up the full Munsell hue scale into 10 sections R, YR, Y, GY, G, BG, B, PB, P, and RP is a way of making the numbers a bit easier to remember than just a number from 0–100, but the precise boundaries of those categories are not in themselves meaningful. The whole point of the Munsell system is that you learn what color the hue number represents, and then it’s predictable, you don’t need to rely on someone’s arbitrary choice for each color term, or the specifics of some printer inks. The 100 hues are uniformly spaced with respect to human perception, and since human color categories are not organized in an orderly way relative to hue perception, it’s inevitable that any rigorously organized system won’t match up precisely with those categories.<p>As for RGB #FF0000, the “R” primary on a computer display is very orange-ish compared to what a human would call the unique hue “red”.