Long answer short: This is expected for multiple reasons. Even though you’re telling the material that its Albedo value is “0” meaning the reflected light has no color and theoretically no intensity, the PBR system in Unreal assumes there is still a Specular response where some light energy is bounced back. Shining a different color light on that surface would dramatically change its value. You can get a deeper black by setting your Specular to 0. It defaults to 0.5 . Or you can try an Unlit material if it needs to be as close to 1:1 as possible.
That’s only one piece of it though. Most colors will be slightly different inside the engine. In some cases you can get closer to the source color by turning off “sRGB” in the color settings,
or setting Compression to Masks or None, but it’s usually not perfect. There is also a Tonemapper that takes all the rendered pixels in the scene and shifts the colors around. You could disable it with a console command but it’s almost never desirable to do that.
Base colors for textures are treated more like colors on a physical surface instead of a color that gets directly displayed on a users screen. There’s physically based materials and lighting, a digital camera, and post processing textures get filtered through.
