In order to evaluate FTIR-based structural response patterns of different kinds of tartary buckwheat, solid residues from three tea products (whole embryo black, whole embryo yellow, and whole plant black) before and after brewing are tested in this study. Gallic acid, quercetin, and bovine serum albumin are used as reference standards for peak assignment to select nine representative absorption peaks. By doing this, peak-height variation rate and peak-area variation rate extracted from samples are combined with FTIR difference spectra as well as multi-index modeling to characterize response direction and intensity of protein-, phenolic-, and flavonoid-related regions. The three samples all show similar overall spectral profiles, while reproducible differences are observed at peaks associated with C==O, aromatic skeleton vibrations, and C—O/C—O—C bands. Whole embryo black samples exhibit higher area-based variation at several polarity-related peaks; whole embryo yellow samples show inconsistent directions between height- and area-based variations at multiple peaks; whole plant black samples present negative area variations in majority, indicating weaker responses. PCA discriminates against structural response patterns among sample types in an effective manner. In conslusion, the proposed workflow, standard classification-based assignment–variation-rate extraction–difference spectra–composite indices–PCA identification provides a practical reference for FTIR evaluation of complex plant-based brewing systems.