Artists have historically employed preparatory sketches on mortar surfaces to assist in the precise arrangement of colored tesserae, effectively serving as a foundational guide for both the composition and stylistic execution of mosaics. These preliminary designs are created by incorporating pigments directly into the mortar substrate, utilizing a method akin to the fresco technique. Referred to as sinopia, this practice was integral to the production of mosaics in antiquity. The present study examines sinopia and associated colored tesserae beneath Roman and Byzantine mosaics in Judea. Using non-invasive analytical methods, mineralogical and chemical analyses indicate that that the colored tesserae contained clays with changing amounts of iron oxides as the major chromophore, showing a wide range of colors, including purple, orange and red. The sinopia beneath showed mainly red and pink colors, through the manipulation of red ochre. This study semi-quantifies the compatibility between the colors of the sinopia and the overlying-colored stones, showing mosaicists had flexibility in choosing the desired color of the stones, although closely working with the mortar painters.