Hello chazmash18
There are a couple of things to know before calling it a bug. Are you playing vs cpu or against real opponents?
If you are playing against real opponents, and they have for example the yellow and black at Signal Iduna, try asking them about it if possible. Ask them if they had to equip the net color that you see.
If you are playing against the cpu, then that's probably down to the difference I mentioned earlier. The cpu opponents don't have customization available, so the stadium uses "default" settings. If you look closely at your customization options in the stadium section, you will see that on net color it does not say "default". In the game it doesn't say anything there, unless you equip a net color consumable. On the app also, it does not say "default". It instead says "none" if you don't equip a net color consumable. So because you are not allowed to use "default" settings in ultimate team, the net color strictly subject to the consumable that you equip. Yellow and black, red and white, and every other color option are included as consumable items, and the only way to use them is to equip them.
Finally, if you can navigate to club ~ stadium ~ structure ~ goal net color, and you can change the color using a consumable, your report will be listed as "not a bug", because in that case this would be normal. The limited customization on authentic stadiums applies to things like pitch trophies and pyrotechnics, while things like seat color, net color, mow pattern and line color remain customizable. So before I can confirm that you should report this as a bug, I would need you to tell me if you are able to apply a net color consumable for your stadium.
All the best to you!