The instruction is the work.

On a white wall draw a rectangle equal to the available field. Divide the field into equal units on a forty-eight pixel measure.

Within each unit draw one straight line: horizontal, vertical, diagonal left, or diagonal right. Assign direction by sequence, not by preference.

Fill only those bounded areas named by the rule: blue first, red second, yellow third, black last. The hand may waver; the system may not.