How to to recognize Brigade Fire Helmet ?

Probably all designs are protected by trademark. If you take a car, it's shape is protected by trademark and copyright, but it can be used as long as it is not the main object in the image, except for iconic cars, like the beetle. However, certain design elements may be protected, as they are contributing to the overall image of the manufacturer. It's not black and white. And sometimes it's a tribute to how aggressive a company defends their design. That's the reason, why you can't put a round button on a smartphone.

The trademarked colour is called Tiffany Blue, and they is probably protected for jewellery. I do not expect a robin egg to be protected. You simply can't trademark nature.

Best is to give the whole story at the beginning of your thread. That avoids confusion. Even if it's not always true, I would assume that those rules are known to the moderators. 

For you, I would suggest you google the helmet in question, to see if your design matches. If it matches, you should not submit. My understanding of this is, that the helmet is a very specific design and anything not matching that would be acceptable.