1.red
rgb(255,0,0)
rgb(100%,0%,0%)
#ff0000
#f00
2.Predefined color name
aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, orange, purple, red, silver, teal, white, and yellow. transparent
3.rgb
4.Hexadecimal/sexadecimal
three-digit:easier to decipher
Six-digit:give you more control over the exact color.
5.HSL(CSS3)
hue,saturation,lightness
6.color vs background-color