In Britain they used to all be colour-coded. Red, Blue, Green, Amber and White. The combination of that plus numbers was enough to differentiate them.
Since then, airspace designation has proliferated as our airways have got more complex. In order to have more options and also to standardise internationally, we have kept the same initials for the colours but have used the international phonetic designator - so Red One becomes Romeo One, and so on.
In the USA, the main prefixes are J (for Jet, ie, high level) or V (for VFR, because you are below US Transition Altitude). Other nations have their own designators.