If you ever see studio shots of the effects model, you see it's a more tan colored, like the Cardassian war ships but techniques of lighting the model for filming made it look more dark grey. It's a tough call because the model it delibrately light to look dark and inividual TV recpetions also come into play. Take a look here under 'Federation Ships' to see how other modelers handled it:
http://www.culttvman.com/trek.html
And here for reference shots of the effects model:
http://www.starshipmodeler.com/trek/ds9tech.htm
I'm currently working one a DS9 Kit myself and I'm not sure what to paint it, either.