So I had a mission that consisted on ammoracking a tank, the reward was a day of premium tank. When I completed the mission on my Helsing, the camouflage button on my tanks that I couldn't apply camo on (Pz IV Anko, Helsing and Kuro) was unlocked (pretty sure because when you recieve premium time the camouflage button is highlighted because the ugly premium camo was available to use), which meant I could ''apply'' a camouflage pattern on a tank that had it's own camo, or at least close enough:
(Notice how the camouflage button is available on the Kuro, when it shouldn't)
(These only apply aestethically, as it doesn't let me purchase any camo, however you can't see any difference on the Helsing because reasons)
I wonder what happens if you try to buy a gold camo, would you pay gold for something you can't use? If you have this bug, try it and tell us what happens.