[Heiku]

I ordered my 598's from Amazon priced 158eur. Visually they seemed fine and they feel like quality product. I've already been using them more than 7 hours and i wasn't very impressed with them, they sound okay. Then just now i decided to ensure they're authentic, when i saw the hologram it wasn't like the rainbow color in the pciture, mine stays unchanged. So i contacted Sennheiser and at the end of the verification i get "Authentication Failed!"
 Could this mean they're fake?

 

[jellofund]

If you bought from Amazon themselves rather than a marketplace seller then I highly doubt you received a fake pair.
 
The fact you got to step 4 makes me think that at least some or all of the information must add up. I know you said that the hologram doesn't change like the one in the picture but did the verification code that Sennheiser displayed on screen match the one on the hologram?
 
If it did then I think you most likely have a genuine pair. In daylight the hologram really doesn't change colour much at all when tilted the way Sennheiser demonstrate. If you shine a torch (or use the flashlight on your phone) directly over it then it should probably work a bit better.