出典の根拠 Tobunken's 彩色DB records 紫土 (purple earth) used in a 752 CE Shōsōin document.
関係 紫土 is a rare manganiferous earth pigment for purple, a material relative of the dye-based 紫.
注意 Earth-pigment context only; it does not supply a measured value for the dye-based purple 紫.
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