Question

give reasons for: we usually do not see bright colours in dim light

Answer 1

Retina is a layer present in the eye and consists of 2 types of cells 
1.rods
2.cones
rods have a pigment called rodopsin which helps us to see in dark and dim light.it detects low intensity of light.so, we can't see bright colors in dim light

Answer 2

rods and cones in our eyes are sensitive to light ,however rods allows us to see in dim light but ignore the detection of light, whereas cones allows us to see color but not in dim light.........