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There are products such as the CS line of worm and lure dips that will stain a soft plastic, I don't have much luck with longevity. Airbrush paints will require the plastic bait be dipped in clear plastic to make it durable but the extra dip also will change the profile and size of the soft plastic beads. There are opaque glitters that work in clear plastic that might be suited to making a bead with a mottled appearance.