Red barrels are wine barrels that were last filled with red wine varietals, and are thus not suitable for white wine storage for risk of turning white wine pink. Red barrels may have been previously filled with white varietals, but at least their last fill was with a red varietal.
Neutral barrels are those that have been in service long enough to have used up most of their oak extraction, thus becoming “neutral” in the oak flavors imparted to the wine. Though slower to imbue the wine with significant oak flavors, these barrels are good, clean wine aging and storing vessels that will impart oak flavors and micro-oxygenation to the wine at a slower rate than newer barrels. Think of the barrel as an oak ingredient in the wine that is consumed more rapidly during the time the first fill is in the barrel and is consumed at a much slower pace after 4-5 years use.
*Ferm barrels are those with a stainless steel “fermentation lock” closure installed in one head. This provides re-sealable access to the barrel to insert whole cluster fruit for barrel fermentation and an exit hole to remove the fruit afterward without having to remove the head.