Check the videos below. When I set RGB colour 0, 111, 255, Rhino will change the preview icon in the materials list to a pitch black. As long as I can see, that issue only happens with this particular colour. The one way to force Rhino render the material icon properly with the desired colour is to switch to the HSV wheel with visible sliders and manually change some of the numbers and then revert to the original number.
I can see that the preview icon renders at real-time, so maybe that blacked out icon situation is related to the real-time rendering in some way? The icon updates properly if I set it to “Sphere”, “Cube” and everything else except “Scene”. It also works if I set a different size for the preview icons.
I also attach a portion of the scene so that you can try whether the material will behave in the same way in your Rhino 7.
No icon preview.3dm (44.5 KB)