My city is very multicultural and I never call my colored homies "niggas". Im white, and it doesnt feel natural at all, they do however use the term nigga on me, which i dont mind, its as you said- a synonym(i think?) for homie, brother, etc.
White Friends of mine do say nigga, and they are not offended by it. I would advice tho to not call a complete stranger afro-american "nigga". They might find it offensive, but in general when you are good with em they do not take offense.