Pythonでopenpyxlを使用してExcelファイルを生成することができますが、グラフに設定できるカラーとして以下があります。
openpyxl.drawing.colors module
しかし、名前を見ても実際のカラーが分からなかったので実際に全てのカラーを設定したグラフを生成しました。
mediumTurquoise

wheat

royalBlue

ltGrey

darkSlateBlue

dkSlateBlue

deepPink

oliveDrab

violet

honeydew

brown

dkOrange

slateGrey

tomato

indigo

khaki

ltSlateGray

azure

mediumOrchid

lightGoldenrodYellow

lightSkyBlue

dkMagenta

lightSlateGrey

ltYellow

mediumSeaGreen

darkGray

dkTurquoise

moccasin

paleVioletRed

purple

slateBlue

dodgerBlue

seaShell

darkOliveGreen

dkOliveGreen

lightSteelBlue

ltGoldenrodYellow

medTurquoise

snow

teal

aliceBlue

dkCyan

lawnGreen

lightPink

aquamarine

dkOrchid

aqua

ltSteelBlue

sienna

blueViolet

mintCream

cornsilk

mistyRose

burlyWood

mediumVioletRed

seaGreen

darkKhaki

powderBlue

dkBlue

antiqueWhite

darkOrchid

floralWhite

darkMagenta

hotPink

darkCyan

darkSlateGrey

cadetBlue

dkRed

greenYellow

medSeaGreen

skyBlue

lightSalmon

medOrchid

mediumAquamarine

chartreuse

dkSlateGrey

linen

medVioletRed

yellowGreen

ltCyan

magenta

darkSlateGray

deepSkyBlue

grey

olive

lightGrey

lemonChiffon

lightBlue

midnightBlue

mediumSpringGreen

lavenderBlush

lightYellow

dkGrey

blanchedAlmond

crimson

oldLace

navy

bisque

cornflowerBlue

dimGrey

ltSlateGrey

ltSalmon

limeGreen

plum

slateGray

white

rosyBrown

ltBlue

lime

gray

salmon

darkSeaGreen

dkGreen

ltSkyBlue

orchid

ltSeaGreen

beige

forestGreen

peru

blue

ltCoral

darkOrange

medPurple

darkTurquoise

darkGoldenrod

whiteSmoke

goldenrod

ltGreen

navajoWhite

orange

gold

green

saddleBrown

lightGray

medSpringGreen

lightSeaGreen

papayaWhip

fuchsia

dimGray

darkGrey

mediumSlateBlue

darkViolet

dkViolet

ltPink

medAquamarine

dkSalmon

maroon

lightCoral

dkSlateGray

coral

lightSlateGray

medSlateBlue

indianRed

darkBlue

ivory

medBlue

dkGoldenrod

paleGreen

thistle

dkGray

firebrick

pink

ghostWhite

lavender

tan

mediumPurple

red

springGreen

steelBlue

sandyBrown

dkKhaki

lightGreen

paleGoldenrod

darkRed

lightCyan

darkSalmon

black

chocolate

ltGray

peachPuff

silver

turquoise

yellow

dkSeaGreen

orangeRed

paleTurquoise

cyan

gainsboro

darkGreen

mediumBlue
