Just because somewhat didn't really think this one through.
Don't let it bother you too much. :P
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Here they all look blonde. So it would represent their "non modified version".
Pink for Pink Hornet.
Red/Orange for Sparking Red.
Blue for Electric Blue.
Green for Winch Green.
We could ASSUME that "stealth black" DOES NOT break this pattern.
So the change in color in the anime is more likely to be an error, or an intentional change. Since the unmodified versions all have blonde hair, the hair color is due to the modifications working on them.
Sanji's hair would have been Black, had the modifications worked properly on him, to keep the same pattern with the others.