Question

What would the colour of a yellow dress be under magenta light?

Answer 1

yellow + magenta= red

Answer 2

Magenta light contains Blue and Red lights.
The fact that the dress appears yellow (when shone upon by white light) means it absorbs Blue light.

Hence, Blue light gets absorbed from the magenta light, leaving Red light only.

Answer 3

@vincent I thought that the dress appears yellow (when shone upon by white light) because it absorbs all colors except yellow.