A
mnemonic used in
astronomy to indicate the classes (or colors) of
stars, which was
OBAFGKM or (O, B, A, F, G, K and M), on a
stellar evolution diagram ranging from hot, early type stars (O) to cool,late type stars (M) class.( Three additional classes (RNS) (or
OBAFGKMRNS) are no longer used. The mnemonic for that one was
Oh Be A Fine Girl Kiss Me Right Now Sweetheart.)
About 200 years ago it was discovered that sunlight is not merely one color, but in fact is made up of bands of hundreds of colors. A much more complete explanation is located under Stellar Spectra, and a nice visual display is shown at Hertzsprung-Russell diagram.