Which technique is correct for changing darkness from top to bottom while keeping the same blue hue in a sky?

Correct answer: Gradually change the pigment-to-water ratio

The governing concept is a monochromatic value gradation. To keep the same blue hue while changing darkness, gradually alter the pigment-to-water ratio: more pigment produces a darker value and more water produces a lighter one. A continuous wet edge helps the transition remain smooth. Using different colour families changes the hue, applying red and yellow breaks the blue scheme, and using one identical mixture cannot create a gradient. Thus, option A is correct.