Here is a great standard flat ui color palette courtesy of Designmodo. We should have a few more up here, so be sure to subscribe so you can receive more flat color palettes!

#1abc9c
Turquoise
#16a085
Green sea
#2ecc71
Emerald
#27ae60
Nephritis
#3498db
Peter river
#2980b9
Belize hole
#9b59b6
Amethyst
#8e44ad
Wisteria
#34495e
Wet asphalt
#2c3e50
Midnight blue
#f1c40f
Sun flower
#f39c12
Orange
#e67e22
Carrot
#d35400
Pumpkin
#e74c3c
Alizarin
#c0392b
Pomegranate
#ecf0f1
Clouds
#bdc3c7
Silver
#95a5a6
Concrete
#7f8c8d
Asbestos