Remember the VP test color chemistry:
Acetoin $+$ $\text{KOH}$ (oxidizer) $\rightarrow$ Diacetyl.
Diacetyl $+$ peptone (guanidine) $+$ $\alpha$-naphthol (catalyst) $\rightarrow$ Cherry Red Complex.
Barritt's A = $\alpha$-naphthol; Barritt's B = $\text{KOH}$.