Here is an excellent guide to color combinations for watercolors.

Recommended  by Rose Margaret Braiden, Santa Barbara Los Padres Watercolor Society founder.  She is in Who’s Who in American Art and Who’s Who in American Women.

REGULAR ORANGE
Rose Madder Genuine
Aureolin Yellow

INTENSE ORANGE
Aureolin Yellow
Rose Madder Genuine

ORANGY ORANGE
Winsor Yellow
Winsor Red

BRILLIANT ORANGE
Winsor Red
Winsor Yellow

(Note that these last two use the same color, but in different order.)

DIRT
Burnt Sienna

COOL VIOLET
Cobalt Blue
Rose Madder Genuine

INTENSE (REDDISH) VIOLET
Alizerin Crimson
French Ultramarine

INTENSE (PURPLISH) VIOLET
Cobalt Blue
Rose Madder Genuine

BLACKISH VIOLET
Cobalt Blue
Indian Red

GRAYED VIOLET
Winsor Blue
Winsor Red

BLACK
Winsor Blue
Hooker’s Green
Alizerin Crimson
Burnt Sienna
Or
French Ultramire
Burnt Sienna
Alizerin Crimson

BROWNISH RED
Cadmium Yellow
French Ultramarine
Cadmium Red
Or
Yellow Ochre
Indian Red

EARTHY BROWN
Cobalt Blue
Viridian

NATURAL BROWN
Indian Red

RIVERBANK
Burnt Sienna
Winsor Blue
(foreground)
Burnt Sienna
Winsor Blue

AUTUMN TREES
Cadmium Red
Cadmium Yellow or Cadmium Orange
Burnt Sienna (suggests dying grasses)

WINDOW LIGHT/NIGHT EMISSION
Cadmium Yellow (saturate it)
DAY EMISSION
Winsor Blue
Burnt Sienna

SUNLIGHT
Alizerin Yellow (glazes)

FOLDS
Cobalt Blue
Rose Madder Genuine  for lavenderish result
If Brunt Sienna added, peak dark

GLAZING

GLAZES ARE:
AUREOLIN YELLOW
OR
ROSE MADDER GENUINE
OR
COBALT BLUE

Equal amounts of
Alizerin Crimson
Rose Madder Genuine
+ . Cobalt Blue
= orangy glaze

Equal amounts of
Cobalt Blue
Rose Madder Genuine
+ . Aurelolin Yellow
= purplish glaze

Equal amounts of
Aureolin Yellow
Cobalt Blue
+ . Rose Madder Genuine
= greenish glaze