I have not done this but have a few friends who make custom backgrounds. They swear by coloring sealent. Most paints and sealents have toxic substances in them. I would assume there is something out there, but will take some searching. Finding a 100% safe way to color the aquarium sealent is much easier. And much like foid coloring will be easy to mix custom colors yourself. They would kill me if i gave away how to achieve this, but hope this hint helps.
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