Color variations and sometimes the frosted appearance of Chinese blue-and-white porcelain produced in folk kilns and recovered in underwater archaeological contexts are usually reported without further investigation. This study focuses on the causes responsible for the appearance of the glaze and blue decorations of late fifteenth-century blue-and-white porcelain from the Santa Cruz, one of the most important mid-Ming shipwrecks discovered in Asian waters. Besides detailed visual observations, chemical composition and colorimetric data were collected on a set of similarly shaped plates showing significant differences in the aspect of the glaze and tones and shades of the blue color. Results from portable X-ray fluorescence (pXRF) analysis confirmed the Jingdezhen origin of the ware and the use of domestic asbolane ores for the Co-based blue pigment. Visual and microstructural analysis has shown that the degree of opacity of the glaze is primarily linked to the crystallization of anorthite, which in some cases has pushed the pigment layer towards the surface, contributing to the development of white-brownish weathering spots. The colorimetric data acquired with fiber optics reflectance spectroscopy (FORS) allowed us to quantify chromaticity parameters and confirm the visual perception of a ‘not-so-blue’ color of the decoration. Without excluding a possible contribution of the underwater environment, the observed variations can be mainly attributed to the ware’s production and more specifically to pigment characteristics, manufacturing processes, and firing conditions even though the connection with these factors is not straightforward and prompts further research and broader discussions. From a historical perspective, it is suggested that the atypical hues are correlated with the progressive switch from foreign to domestic pigment sources during the Chenghua period (1465–87 CE) and the subsequent technological adaptations required by an ever-increasing demand for Chinese blue-and-white porcelain at the turn of the sixteenth century.

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