XFCE actually just uses the (admitingly beautiful) default GTK palette, so the colors aren't specific to the WM. Gray is the simple default, which is actually a pretty good match for XFCEs "no bullshit"-philosophy.
Preview#ffffff
Files
Trash
Files
Documents
10:42
All colors46 · click to preview
Gray
#808080
Powder Blue
#99c1f1
Clear Sky Blue
#62a0ea
Azure Blue
#3584e4
Royal Blue
#1c71d8
Cobalt Blue
#1a5fb4
Mint Green
#8ff0a4
Meadow Green
#57e389
Emerald Green
#33d17a
Jade
#2ec27e
Verdant Green
#26a269
Primrose
#f9f06b
Lemon Yellow
#f8e45c
Sunflower
#f6d32d
Golden Yellow
#f5c211
Amber Gold
#e5a50a
Apricot
#ffbe6f
Mandarin Orange
#ffa348
Vivid Orange
#ff7800
Copper Orange
#e66100
Rust Orange
#c64600
Coral Red
#f66151
Poppy Red
#ed333b
Vermilion
#e01b24
Carmine Red
#c01c28
Burgundy
#a51d2d
Lilac Bloom
#dc8add
Mulberry Purple
#c061cb
Royal Purple
#9141ac
Deep Purple
#813d9c
Grape Purple
#613583
Sandstone
#cdab8f
Caramel
#b5835a
Chestnut
#986a44
Cocoa
#865e3c
Espresso
#63452c
White
#ffffff
Porcelain
#f6f5f4
Pearl Gray
#deddda
Silver Gray
#c0bfbc
Warm Ash
#9a9996
Pewter Gray
#77767b
Charcoal Gray
#5e5c64
Graphite Violet
#3d3846
Ink Black
#241f31
Black
#000000
White
KNOWN USES14 palettes · 1985–2016
19852016
First in Amiga Workbench 1.x, 1985. Recurs across 14 palettes through ReactOS 0.4.x, 2016.