What is the benefit of working on lighter areas first while filling colours?

Correct answer: Smudging from dark colours is reduced

A useful colour-filling principle is to establish lighter areas before applying stronger dark colours. Dark paint can easily transfer to nearby regions or dirty a light area when the brush or hand moves across the surface. Beginning with light sections reduces this risk and makes later dark accents easier to place accurately. It does not make paper dry, enlarge the brush, or erase the drawing lines, so A is correct.