A monochromatic color scheme is a one-color color scheme. However, the
color can be neutralized by adding its complement to lower the intensity
of the color. Black and white can also be used to darken and lighten the
value of the color.
In this color scheme any two complements, all the semi-neutrals and the
neutral they produce can be used. Black and white can also be used.
Any three adjacent primary, secondary, or tertiary colors are analogous
colors and can be used in an analogous color scheme. Each can be neutralized
by use of its complement, and black and white can be used. Analogous colors
"harmonize" well and produce a definite mood to a composition.
Any three adjacent primary, secondary, and tertiary colors can be used,
plus the complement of the middle hue. This complement is used subordinately
and to produce semi-neutrals of the three colors while maintaining color
harmony. Black and white can also be used.
Any three colors equidistant around the color wheel form a triad
and can be used in this color scheme. Semi-neutrals are mixed
using two of the colors in the triad and the third can be added
to further neutralize the pair. Black and white can also be used.
Color harmony is assured if you choose a color scheme and
stay with it.