There are many ways to detect explosives through senses of sight (general appearance and color), and touch (texture, weight per volume).

FORBES: Young Juvenile Makes An Explosive With Baby Powder And Other Tales From The URI Explosives Lab

If painting the colors back in sounds like hard work, there's also an automatic fill tool to detect edges and colors and either fill in or remove the color automatically from a selection (although we had mixed results using this tool).

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