Here's a bit of a clever cap.
This hat doesn't even have Sonic on it at all. It's plain blue, with just a ring monitor sitting on Green Hill Zone grass for it's main design. The bill of the cap has GHZ 'dirt' in the classic checkered browns pattern. This design is instantly recognizable to any Sonic fan, but it is otherwise non-obvious, making it a bit more subtle of a hat. That green dot there on the brim isn't part of the design, it is a sales ticker. Cap may be found in some Spencers' Gifts stores/possible.