A neutral flame is the hottest flame. Your flame is made up of "candles" at the base of the torch, and you want each candle to be about the same length; 1/4" to 1/3" or so, primarily rounded blue with a small slightly fuzzy whitish-yellow tip. You don't want to see any elongated yellow candles, or a strongly elongated center candle, because that means you have a reduction flame. It's OK for the center candle to be just slightly longer than the outer candles.
A reducing flame has more propane than the usual neutral flame, it's a cooler flame. A reducing flame has longer yellow fuzzy tips to the candles. The flame will look fatter than usual and the candles coming out of the torch will look very long. When you put a rod of ivory or white, (turquoise, yellow and other colors do this as well), you will get a brownish haze on the end of the heated rod. To remove an unwanted brownish stain on a bead or rod, if you DO NOT really want to use a reducing flame but are accidentally using one, turn down the propane on your torch then re introduce the bead or rod into the flame and the brown stain will burn off. If you can't turn down the propane on your torch, turn up the oxygen on your torch. A reduction flame also helps bring metallics to the surface.
An oxidizing flame has more oxygen than usual. An oxidizing flame has rounded end blue candles with no yellow/white tips to them. The further away from the torch head you work, the more oxidizing the flame. Oxidizing flames are good to use on striking colors. Metallic leaf also likes a slightly oxidizing flame.
Reducing flame on torch at Harrach Glass