If I recall, Speedplay pedals used to to come with the cleat screws already blue-dipped. Based on that, it seems like blue would be a reasonable choice.
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RIP 01/08/25...2022 Trek Supercaliber, 2018 Storck Fascenario.3 Platinum, 2018 Trek Procaliber SL Singlespeed, 2017 Bear BR1, 2003 Time VX Special Pro, 2001 Colnago VIP, 1999 Trek 9900 singlespeed, 1977 Nishiki ONP