Grape pomace is a major by-product of winemaking with increasing potential for valorization; however, the role of stabilization as a determinant of its chemical and functional properties remains insufficiently resolved. This study compared three stabilization strategies: convective drying, freezing, and microwave-assisted processing applied to grape pomace from ten temperate climate grape cultivars (red and white). Phenolic composition, lipid-related compounds, and color attributes were assessed using HPLC, LC–MS and GC–MS, and responses to stabilization were evaluated using mixed-effects models accounting for cultivar background. Stabilization strategy exerted a clear and non-neutral influence on pomace properties. Freezing caused a pronounced reduction in lightness (L*), independent of phenolic concentration, indicating dominant matrix-level optical effects. Convective and microwave-assisted drying yielded comparable lightness, while microwave processing enhanced the extractability of lipid-related compounds. At the fraction level, phenolic composition was primarily governed by cultivar, with stabilization inducing only modest changes in total procyanidins and anthocyanins. In contrast, compound-resolved analyses revealed strong structure- and cultivar-dependent responses, particularly among phenolic acids, underscoring the limitations of aggregate phenolic metrics. This study indicated that stabilization is not a chemically neutral preparatory step but a defining procedure that conditions matrix accessibility, compound detectability, and visual properties, with direct implications for grape pomace valorization strategies.