Colour

The governing colour-theory concept is simultaneous contrast. When highly saturated complementary colours of similar lightness are placed close together, the eye compares their opposing colour signals and the boundary may appear to shimmer or vibrate. Neutral grey, a single colour and completely erased boundaries remove the strong opposition needed for this effect. Therefore option A is correct, although artists should use the vibration deliberately because it can reduce visual comfort.

In art and colour theory, saturation describes the purity or intensity of a colour. A highly saturated colour appears vivid, strong and clear, while a low-saturation colour looks duller, softer or closer to grey. Therefore, option A is correct. Shape size, paper thickness and the amount of perspective concern other visual or material properties and do not define saturation.