do you have any lights in your game? I think what you might be seeing is because of lighting. Alternatively, I think you can set unlit on the material to true https://docs.rs/bevy/latest/bevy/pbr/prelude/struct.StandardMaterial.html#structfield.unlit
A material with “standard” properties used in PBR lighting Standard property values with pictures here https://google.github.io/filament/Material Properties.pdf.