Without a ref pic, it is hard to give specific info but I always recommend adding wefts of whatever color streak/chunk you need over trying hair chalk, color sprays, etc. Even using a sharpie to color in small streaks (if that is what your character has?) would be acceptable and effective. With either of the aforementioned methods, you know the color cannot be sweat off, brush off, wash off, or bleed off when using styling products (do a swatch test if your styling product has a lot of alcohol though if you use sharpies).
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