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The same plastic material with Nd=1.6 (left) and Nd=30 (right)
A word on "color" when creating materials
When you specify a color via the UI, Maxwell takes that RGB input and converts it into "spectral space", meaning it treats the "color" values as wavelengths of electromagnetic radiation, having different amounts of energy. The range of possible energy values reflected off an object or transmitted through it, is much greater in the real world than the 0-255 range of an RGB color picker. It is therefore important to remember that when you specify a color, having a certain brightness, you are also defining how much energy that color has, not just its hue. A brighter color will have greater energy than a dark one. This is important both for creating reflective solid materials, and transparent materials.For example a material with the same hue of red but with varying values will produce varying degrees of transparency.
It will be much easier to control the material parameters in Maxwell if you separate in your mind the hue of a color and it's value (brightness). It will be easier if you choose the HSV (Hue, Saturation, Value) mode of your color picker if the UI allows it (The Maxwell Studio color picker has this mode).