**Core Concept**
Color perception and cognitive development in children are closely linked to brain maturation, particularly in the visual cortex. By around 4 years of age, children typically demonstrate an understanding of basic color categories, such as red, blue, green, and yellow.

**Why the Correct Answer is Right**
The correct answer is associated with the development of the visual cortex, specifically the ventral pathway, which is responsible for processing color and form. This pathway undergoes significant myelination and synaptic pruning between 2-4 years of age, allowing for improved color perception and categorization. Children at this stage can identify and name basic colors, demonstrating an understanding of color concepts.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because 1-2 years is too early for children to demonstrate consistent color naming skills, as their brain is still developing the necessary connections for color perception.
**Option B:** Incorrect because 5-6 years is too late, as most children have already developed basic color categorization skills by this age.

**Clinical Pearl / High-Yield Fact**
Children's color perception and categorization abilities can be influenced by factors such as visual acuity, cognitive development, and exposure to colors. A child's ability to name colors by the age of 4 is a general marker of typical cognitive development, but should not be used as a sole indicator of intelligence or developmental disorders.

**Correct Answer: D. 4 years.**