First of all they take very good care of their skin since their image is their bread and butter. Regular facials, a good diet and lots of water go a long long way toward having good skin, which is the base of the celebrity glow.
Other than that Benefit Highbeam gives a great sheen on the cheekbones and is used by a lot of celebrities. It doesn't add colour as such, just a subtle sheen. There are many other products out there that do the same thing, but Highbeam is the celebrity fave.