Most of us love fireworks during Diwali. But these bright and colourful crackers have a dark side It's the toxic metals that give fireworks the beautiful colours we find magical The red colour of a firecracker comes from Strontium, a metal that causes bone growth problems in children Barium, which gives fireworks the green colour, is harmful to the nervous system, the heart and can cause tremors, weakness, anxiety, shortness of breath and paralysis.