Borax combines with metal salts to generate metal borates or metals with distinct colors.
So this test is only useful for colored salts.
The emergence of color on a bead of borax in a loop of platinum wire held in the reducing and oxidizing parts of a bunsen burner flame indicates the presence of metals.
Example
In the Borax bead test, a copper salt gives red in cold, and colorless in hot.
In the Borax bead test, a nickelate salt gives a grey color.
Hence, the borax bead test is given by colored salts.