Monochromatic

Removing hue contrast forces every other decision to work harder. Composition, value and shape have nowhere to hide, which is why a monochromatic picture is a good test of whether the underlying image is any good.

It also reads as one thought. Nothing in the frame can be about a different idea from anything else, because everything is made of the same colour.

Pick one hue and build five values from it: near-black, dark, mid, light, near-white. Those five are the entire palette.

Use the whole range. The single most common failure is three values clustered in the middle, which produces a flat, foggy picture with no structure.

Keep the hue constant as the value changes. Letting a dark blue drift toward purple quietly turns the scheme analogous.

Use it for mood and for focus: a portrait series, a moody gig poster, an architectural study, a memorial, anything where colour would be a distraction.

Leave it alone when something needs to be found quickly. With no hue contrast there is no way to make one element jump.

Achromatic has no hue at all, only greys. Monochromatic keeps one, and that one colour carries all the feeling.

Analogous allows neighbouring hues, which gives more life. If a second hue has crept in, the scheme has moved.

Picasso's Blue Period is the famous case, and it works because the value range is enormous rather than because the blue is interesting. Cyanotype does the same thing chemically, and duotone editorial printing did it to save an ink.

A second hue appears in the shadows, usually a warm brown. Say one hue only, at every value, and no warm accent.

The five values bunch in the middle and the picture goes foggy. Say the range runs from near-black to near-white.