The top is red, the right is yellow, and the bottom is blue.
Apparently, when the left section is painted, it will be a larger area than each of the other three colours.
What colour is the left section?
The outside of the square is irrelevant.
The answer is there within the square.
The area of each of the triangles is the same, so you need to increase the size by looking elsewhere.
The lines inside the square are quite thick.
If you paint the section the same colour as the lines, its area will increase by the size of the lines.
The lines are black.
The answer is '6'.