Objects consist of outlines and optionally, fill. If there’s fill, it doesn’t matter what the color is, it will result in the object being treated differently than if it wasn’t filled. When you engrave, this can make a big difference since unfilled will engrave an outline and filled will engrave the entire object. But if all you want to do is engrave the outline, you don’t actually want engrave anyway. Engrave is “rasterized”, so a vertical solid line will be made up up many tiny stacked dots. This can result in “ratty” outlines. But if you use Score instead, whether the object is filled or not doesn’t matter. The laser will follow the outline like it was cutting, except it will run at a lower power that only removes a little bit of the material.
Score is actually what you want for this. As a bonus it’ll be substantially faster than engrave would be…