star - How to calculate B-V colour index value percentage difference

$B-V$ corresponds to the base 10 logarithm of a flux ratio.

$$B-V = -2.5 log left(frac{f_B}{f_V}right)$$

So trying to guess what you are trying to calculate, it is the percentage change in the blue to visible flux ratio?

In which case the percentage change is

$$p = frac{ 10^{-(B-V)_2/2.5} - 10^{-(B-V)_1/2.5}}{10^{-(B-V)_1/2.5}}times 100$$

A percentage change can of course be negative.

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