Hello chazmash18
I'm not sure, but it could be that all those net color options besides "none", which is white, are consumables that must be earned or bought, and then equipped. The opponents you face may be fond of the stadium net color they choose in real life, and equip the consumables to match.
Can you check what your current net color is set to in the stadium section of ultimate team?
Also, you can test this by changing net color, then change stadium to another that traditionally has it's own. For example, switch to cyan and then switch to, Anfield, which should have red nets. If the nets remain cyan, then this is probably the explanation.
All the best to you!