Barium oxalate

Barium oxalate, a barium salt of oxalic acid, is a white odorless powder sometimes used as a green pyrotechnic colorant generally in specialized pyrotechnic compositions containing magnesium.

Though largely stable, it can be reactive with strong acids. A mild skin irritant, the substance is considered toxic when ingested, causing nausea, vomiting and renal failure.

External sources

 * rec.pyrotechnics FAQ
 * MSDS