# How do I calculate B-V to make a Hertzsprung Russell diagram?

I am reading my textbooks and a few "Astro101" websites, but I cannot find a final answer to this:

In the $$B-V$$ colour index that is plotted in an H-R diagram, are $$B$$ and $$V$$ the (observed) apparent magnitudes, or should they be corrected for extinction first, i.e. with known $$A_V$$ and $$A_B$$ values?

If you can correct them because you know A_V and A_B (which can be hard), it's probably better, although I don't think there is a general rule here.