How close to the element are the darker ones?
I helped my buddy powder coat an aluminum trailer that gets pulled behind a motorcycle. The lid for it was hi ged kind of like a car's hood. The heating elements for my buddy's walk in PC oven are near the floor/bottom. We got the lid wheeled in there but the way it was hanging one corner was too close to the element. So we wheeled it out after about 15 minutes thinking it was done. It was supposed to be orange. We shine a drop light on it inspecting every square inch, we get to the bottom and see where it is more like a brown. "FFFFFUUU...!!!"