A hot air balloon is not solid, a bit of transparency should be ok.  If not the light can't be seen.
You could try a hollow balloon and make the inside an even color to help with the light.  The outside can be multicolored but have orange or something as the inside color.  
I haven't tried this but it may be something along those lines.

I just tried several materials with ambient at 100 with no sun and a black background and it was bright.  
What material are you using?