Here is a quick note of generating gltf file without losing colors on the original step file. Since I am converting cad files from my company, I have avoided using online conversion tools.
First, you got to have a step file.(YES)
you can find some sample files here: https://grabcad.com/library
2. Import step file, and then export to dae using freecad https://www.freecadweb.org/wiki/Download
3. Lastly, conver dae to gltf with the following tool:
You may test out your gltf with cesium.js
download the latest release here https://cesiumjs.org/downloads/
And then run cesium locally with node.js.
Follow the 3D models example to replace the url of sample 3D models to your gltf.