why do most highlights on plastic objects look white while those on gold metal look gold
Answers
Answer: Specular reflection from polished gold is gold colored. Due to the interband transition in gold, green and blue light gets strongly absorbed, biasing the reflected spectrum toward yellow and red. This has nothing to do with diffuse scattering. If you look at a polished gold mirror (of which there are many in my lab), the image you see of objects illuminated by white room light is colored gold.
Highlights that you might see reflected from dielectrics are white because dielectrics are relatively dispersionless (i.e. the refractive index is more-or-less independent of color) throughout the visible range. Gold is decidedly not dispersionless in the visible.
Answer:
Yes, specular reflection from polished gold is gold colored. Due to the interband transition in gold, green and blue light gets strongly absorbed, biasing the reflected spectrum toward yellow and red. This has nothing to do with diffuse scattering. If you look at a polished gold mirror (of which there are many in my lab), the image you see of objects illuminated by white room light is colored gold.
Highlights that you might see reflected from dielectrics are white because dielectrics are relatively dispersionless (i.e. the refractive index is more-or-less independent of color) throughout the visible range. Gold is decidedly not dispersionless in the visible.