The aim of this study was to confirm the use of an admixture of natural earth and a second unusual Al-rich red pigment in the imprimaturs of paintings that date back to 1600–1660, painted by some of the most outstanding Valencian Baroque painters. For this purpose, microsamples excised from six paintings were prepared as cross-sections to be analysed by optical microscopy (OM) and field emission scanning electron microscopy-X-ray microanalysis (FESEM-EDX) combined with Fourier Transform Infrared (FTIR) spectroscopy, microRaman spectroscopy and voltammetry of immobilized microparticles (VIMP). Two possible red pigments, red lake or synthetic Mars red could be used by these painters in red imprimaturs mixed with natural earth pigments. This particular technique, identified for the first time in seventeenth-century Valencian paintings of known authorship, is a novel finding than can help in authentication studies of unknown Baroque paintings.