Some parallel watercolour lines are dark and others light. Which control is lacking?

Correct answer: Consistency of brush load and paint dilution

The governing concept is consistency of paint load and dilution. A brush holding more pigment or a thicker mixture produces a darker line, while a brush with less pigment or more water produces a lighter line. Therefore, repeated dark-light variation usually shows that the brush was not reloaded consistently. Even paper wetness can affect spreading, and equal spacing or brush angle affects placement and shape, but none is the main control responsible for value changes in these parallel strokes.