If you are talking about power compacts, 50/50 means each part of the bulb is a different color. It is one bulb with 2 tubes. Both tubes go on and off at the same time as, again, it is one bulb. You can get many mixtures of 50/50 like 6700/10000, 10,000/7100, 10,000/actinic etc. There is no standard mix but a popular mix is the 10,000/actinic.