When you make a "special" firework charge (anything but one plain color) and place it in a crafting area, (inventory or crafting table) and make it in to a firework, it will loose its properties and will be a plane colored one. I don't know how long it has been In the game for, but it is still in MCBE in 1.17.2.
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We're tracking this issue in MCPE-70383, so this ticket is being resolved and linked as a duplicate.
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