A few weeks ago I started to assemble and paint a Build+Paint Land Speeder for the small Sharpie-painted Ultramarines force.
Over the weekend I completed the Land Speeder, and I think it turned out well for the time and effort spent. The last few steps were to dip the model, apply the decals, glue in the drive and gunner, re-apply dipping media over the decals. I finally applied a matte spray to tone down the reflection of the dipping media to complete the model.
There are a few touch-ups needed to be made with a fine Sharpie pen, (in areas where I still see some blue where it shouldn't be). As a proof of concept, it was a success. The metallic Sharpie pens worked great and saved a ton of time painting those colors.
I probably won't paint any miniatures solely with Sharpie pens alone going forward, but for models with some metallic colors, I'll be sure to pull out the Sharpies - at least for easy to mark areas.