If the sum of diffuse and specular color in Stellar exceeds physically plausible values they get automatically corrected by following common energy preservation rules.
Materials like a highly reflective white plastic need special care during preparation.
When using a pure white diffuse color (RGB: 255/255/255), make sure to use a moderate gray value as reflection color (e.g. RGB: 128/128/128).
As the result is a combination of multiple colors it is up to the user to find a good balance between them depending on the desired visual appearance.
Please compare the images below.