This appears to be colored light emitted by the skybox.
When you don't want the skybox to act as a light source with the color of its textures, then
- Open the lighting window (Menu entry Windows->Rendering->Lighting)
- Go to the "Environment" tab
- Change the "Source" under "Environment Lighting" from "Skybox" to "Gradient" or "Color". You can then select the color of the ambient light. If you don't want any ambient light at all, set the ambient color to pure black.
It is also possible that this isn't actually ambient light, but actually a reflection of the skybox. This would explain why it only appears on "shiny" objects. When you want reflections on specular materials to reflect the actual environment around them, then make sure you scatter some reflection probes around your scene. But keep the performance concerns in mind.