Question

what colour would a green dress,red lipstick and white shoes appear under yellow disco lights and why

Answer 1

Dress: still green
Lipstick: orange
Shoes: still white
Why: Because yellow and green (mixed together) will still be green because green is too dark to mix with green.
The red lipstick would be orange because if you mix yellow and red paint together you would get orange.
The white shoes would be yellow because white reflects color.

Answer 2

bridget: Please don't add to the world's respository of ignorance.

Green under yellow is black,
red under yellow is black
white: appears yellow

Why: green absorbs yellow (that is why it is green, all other colors have been absorbed.)
One caveat. Few "colors" of pigments are pure. Green pigment may have some yellow mixed, and red, may have some yellow in it.