Locating a star cluster on a Hertzsprung-Russell diagram with other color index Recently I took pictures of the NGC 884 cluster in the B and I bands and selected some 30-what stars from them. I've reduced the data to a colour-magnitude diagram (absolute magnitude in the I band $M_I$ vs. (B-I) colour index). I am now asked to designate where the stars I've selected from this cluster are located on the Hertzsprung-Russell diagram and the approximate age of the stars.

My problem is the following: any HR diagram that I can find is either expressed in effective temperature instead of colour index, or is in the colour index (B-V). Also I can only seem to find empirical formulas for the effective temperature that use the (B-V) colour index, not the (B-I) one.

Does anyone know where to find an HR diagram that uses the (B-I) colour index?
With an absolute magnitude of zero and a colour (any colour) of zero, your stars are around spectral type A0 (by definition). The only thing that can confuse this is reddening/extinction.

If there is appreciable reddening/extinction, then your stars could be more luminous and hotter (O/B stars), but there isn't any way to tell from your data.