This is one of the paradoxes of painting that a large area of flat colour seldom appears as colourful, or indeed, as realistic, as one that is textured or broken-up in some way. Oil painting is noted for it. See (RJM's Sea Wall or his Wolf-Hound). Study the Impressionists School of Painting. (Figure 5)
With water colour, it is achieved as a dry brush drag, using the troughs and
crests (raised tooth) of the paper. They are colours which contain all three
primaries but when in equal proportions create dull browns and greys. By varying
the proportions can produce subtle and beautiful colours.