Some fireworks contain a mixture of metal ions. Why is it difficult to identify the metal ions from the colour of the flame?

When a mixture of metal ions is heated in a flame, each ion emits its own characteristic colour of light. However, our eyes perceive these as a single, combined colour. This makes it difficult to distinguish the individual components. Furthermore, some metal ions produce a much more intense flame colour than others. For example, the bright yellow flame of sodium can easily overwhelm or mask the paler colours of other ions like the lilac from potassium, making them impossible to see.