श्यावो नीलश्चाप्राप्तकः ॥ कZ_०२.१३.२७ ॥
śyāvo nīlaś cāprāptakaḥ
If it is dark-brown or bluish, it is unacceptable (not to be received).
It means the treasury/mint should not accept such metal as standard bullion—preventing loss from impure or debased gold.