Add a B - V color index label in the details dialog, that displays B -
V color index whenever we have the data to do so. However, some
polishing needs to still be done.

Besides, it's very weird right now, because we have B and V data for
faint stars, but not for bright ones! So you can see B - V color index
for 15th mag stars, but not for 3rd or 11th mag stars.

FEATURE