Final answer:
A green dress under blue light may appear duller or even black, as it can absorb parts of the blue light spectrum and reflect less blue light than a blue object would.
Step-by-step explanation:
When a blue light shines on a green dress, the dress would appear as a different colour due to the interaction of the light with the pigments in the dress. The color we see is the result of the light that is reflected off of objects and not absorbed. Since a green dress reflects green light under white light, when illuminated by blue light, the blue component that is not absorbed by the dress might be reflected, but because green objects typically absorb parts of the blue spectrum, the dress might appear as a duller green or even black if it absorbs all the blue light. The perception of color is dependent on the light source and the reflective and absorptive properties of the object being viewed.