You can tell something from the brightness and color of a star,
but there are complications.
•The
color gives you an approximate temperature.
•It is
hard to tell the distance from the brightness, because the size is also quite
variable:
•A
small blue star can be as luminous as a larger red star. A blue star can be the same
brightness as a red star if they have the same size but the red star is
much closer.
•If you
get the distance from parallax, then you can tell the size from the color and apparent
brightness.
•Since
most stars are many light years away, their parallaxes are small fractions of an
arcsec.
•Observing
from space helps with this.
•Their
sizes are generally too small to be measured directly (much smaller
fractions of an arcsec).
•Interferometry
will help with this.