Hey, thanks!
I’ve used a Vector Parameter instead of the “Base Color Hue” Scalar Parameter in the material instance to save colors in the picker, but it still works in 0-1 space. It helped a bit since I only needed a few specific colors, and you can save the colors in themes in the color picker (although since it’s still 0-1 space, it’s just grayscale values in the theme previews). I just knew the first color in the theme was red, the second was blue, etc.
As far as mixing decal colors, I’m not sure you can do that and get accurate results with just with the material editor. You can copy all the nodes, rename all the parameters so they aren’t the same for both textures, change the copied texture, and then use add or lerp nodes to combine the base colors, normals, and opacity together in order to get 2 decals “mixing” with each other rather than the default where one is completely over top of the other. It doesn’t mix colors accurately though, and this would only create a new material, it wouldn’t work dynamically in something like a particle emitter.
You could maybe use particle instance parameters to change the colors, but I’ve never attempted to mix colors dynamically. Would be interesting to get something like this working.