Question

why we usually do not see bright colours in dim light

Answer 1

according to my view this is physics . the light rays will not reflect more to our eye in dim light . so may be this happens . 
hope it helps .
if anyhing wrong i m sorry .
thank u

Answer 2

our eye contains 2 types of cells called rods and cones. these cells are present in our eyes in them rods are for detecting the dim light with black and white colors and the cones are for detecting the bright lights with colors. these rods have a pigment called rhodopsin. so when the dim light is taken as stimuli from our eyes they can detected as  the dim light and our rods cells would from an image in black and white. so we cannot see the bright colors in dim light.

hope so my answer is useful................^_^  !!!