There are two ways that flame tests can be invalid. First is a false positive.The classic example of this is the contamination of almost any sample with sodium ions which gives an intense yellow line even when the sodium is only present as an impurity.
Flame Test via kwout
The other is a false negative. Here again sodium ion is often the culprit that masks other emissions unless the emissions are resolved spectroscopically rather than simply visually. The eye is easily misled by overlapping emissions.
The flame test is a fun and useful analytical technique to help you identify the chemical composition of a sample based on the way it changes the color of a flame. However, interpreting your results can be tricky if you don't have a reference.
There are many shades of green, red, and blue, usually described with color names you wouldn't even find on a crayon box!
The color you should expect will depend on the fuel you are using for your flame and whether or not you're viewing the result with the naked eye or through a filter.
It's a good idea to describe your result in as much detail as you can. You might want to take pictures with your phone to compare results from other samples. Read more at ThoughtCo.com.
For steps on how to do a flame test, click here.