I might know what you are talking about, it was a galaxy painted falcon with colors composed of mostly different shades of purple, the most predominant was 'clouds' of a light purple, then the tank was covered with many stars to finish off the galaxy bit. I remember it was absolutely gorgeous, and I saw it was part of an army painted that way, from a member of Advanced Tau Tactica, as that is where I first saw it. I think it might have been done by Jutami or P'Shar's Rifles, frequent members of the forum. You might want to PM Eiglepulper, as I am pretty sure he is a big shot over at ATT and a great poster there, he might know better.
Other than that, to paint it? You would need to do extensive blending of multiple purples for sure, to achieve different lighting from the space 'clouds'. It would definately be difficult to paint, but the result would be amazing.
Good luck!