Finding: Use a grid as a physical palette (save startup file). Neither Blender nor Godot seem to allow specifying a palette width. Eyedropper is another issue*.
In Blender must use unshaded rendering to get accurate colors, if you use color attribute for emission the color is still de-saturated in rendered mode but does sample the correct color with eyedropper.
I do wonder if there are low-poly-oriented options (be it model software or Godot plugins) that handle palettes better.
(older finding: use lambert_wrap diffuse)
Status: I have a custom shader that can use transparent areas to define glow area (glow defined by per-instance shader parameter, similar parameters work for textmesh or csg or custom effect animations etc). Metal and plastic materials are alright too, need to make more models to test more.
Sort of disliking Godot’s shadows, but not sure if I want to go fully unshaded. I would be much happier doing so if I could darken it per-object or per-vertex based on lighting conditions (diffuse shadows only), but I don’t think there is an easy way to do so. Especially needing to use pixel functions.
Stencil shadows would probably be what I want, haven’t tested anything like that though.
In the weeds: Not sure how I want to handle transparency if at all. I was hoping specular/clearcoat could be visible on transparent faces, if that’s possible I don’t know how.
The fishbowl’s flipped-face approach does look better (more like water) as unshaded, though.
* Blender has a shortcut for it but only if your mouse is in the right place
EDIT: If it isn’t obvious, this is about vertex colors.
Image with some of the things I mentioned, plus I made a pane of glass. I wanted a no-depth-test effect though that didn’t work as desired, alpha/multiply aren’t too bad for this but the effect in the screenshot is proximity fade (which I like the look of and think could have an interesting eyeglasses effect like I was thinking). The shine faces have the unshaded material assigned instead (which is why it casts a shadow).
I forgot to say: I would be interested in knowing if custom properties in Blender could be used for shader parameters.
EDIT: Further on why I don’t want full unshaded: I’m leaning on all sorts of material types for extra color range (/special effect). My palette is 28 colors made with 4-value ramps, so each hue the darkest was a choice of a shadow color or a midtone. Metallic gives darker colors, too.
I have also changed many of the settings for a more neutral/default look. I will probably change light temperature and ambient color in a complete scene before trying different tonemaps again. With a soft-low shadow setting I like angular distance (directional light setting) again for self-shadows (too high messes with specular, though).